Why Does StartCopyFromUriAsync Fail When Copying Between Two Local Storage Accounts?

The error already hints at the problem.

You will need to authenticate against the source blob. Unless the source blob is public available you will need to use a SAS token for example to add authentication information. Add a method like

CSHARPCopied
public static async Task<Uri> CreateServiceSASBlob(
    BlobClient blobClient,
    string storedPolicyName = null)
{
    // Check if BlobContainerClient object has been authorized with Shared Key
    if (blobClient.CanGenerateSasUri)
    {
        // Create a SAS token that's valid for one day
        BlobSasBuilder sasBuilder = new BlobSasBuilder()
        {
            BlobContainerName = blobClient.GetParentBlobContainerClient().Name,
            BlobName = blobClient.Name,
            Resource = "b"
        };

        if (storedPolicyName == null)
        {
            sasBuilder.ExpiresOn = DateTimeOffset.UtcNow.AddDays(1);
            sasBuilder.SetPermissions(BlobContainerSasPermissions.Read);
        }
        else
        {
            sasBuilder.Identifier = storedPolicyName;
        }

        Uri sasURI = blobClient.GenerateSasUri(sasBuilder);

        return sasURI;
    }
    else
    {
        // Client object is not authorized via Shared Key
        return null;
    }
}

And modify your code to

CSHARPCopied
    var sourceAccountName = "devstoreaccount1";
    var sourceAccountKey = "Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==";
    var sourceBlobEndpoint = $"http://127.0.0.1:10000/{sourceAccountName}";

    var destinationAccountName = "devstoreaccount2";
    var destinationAccountKey = "Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==";
    var destinationBlobEndpoint = $"http://127.0.0.1:10000/{destinationAccountName}";

    var sourceConnectionString = $"DefaultEndpointsProtocol=http;AccountName={sourceAccountName};AccountKey={sourceAccountKey};BlobEndpoint={sourceBlobEndpoint}";
    var destinationConnectionString = $"DefaultEndpointsProtocol=http;AccountName={destinationAccountName};AccountKey={destinationAccountKey};BlobEndpoint={destinationBlobEndpoint}";

    var sourceContainerName = "srccontainer";
    var destinationContainerName = "destcontainer";

    var sourceBlobName = "myblob";
    var destinationBlobName = "destblob";

    var sourceBlobServiceClient = new BlobServiceClient(sourceConnectionString);
    var destinationBlobServiceClient = new BlobServiceClient(destinationConnectionString);

    var sourceContainerClient = sourceBlobServiceClient.GetBlobContainerClient(sourceContainerName);
    var sourceBlobClient = sourceContainerClient.GetBlobClient(sourceBlobName);
    Uri blobSASURI = await CreateServiceSASBlob(sourceBlobClient);

    var destinationContainerClient = destinationBlobServiceClient.GetBlobContainerClient(destinationContainerName);
    var destinationBlobClient = destinationContainerClient.GetBlobClient(destinationBlobName);

    await destinationContainerClient.CreateIfNotExistsAsync();

    Console.WriteLine($"Starting copy from {blobSASURI} to {destinationBlobClient.Uri}");

    var copyOperation = await destinationBlobClient.StartCopyFromUriAsync(blobSASURI);
    await copyOperation.WaitForCompletionAsync();

    Console.WriteLine("Copy completed");

More background, including on how to generate a SAS token for a container instead of a blob can be found here

unable to StartCopyFromUriAsync a blob between 2 storage accounts

You can use the below code that will copy a file from one Azure storage account container to another Storage account container in the local emulator setup using azurite.

Code:

CSHARPCopied
using System;
using System.IO;
using System.Threading.Tasks;
using Azure.Storage.Blobs;

class Program
{
    static async Task Main()
    {
        string sourceAccountName = "devstoreaccount1";
        string sourceAccountKey = "Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==";
        string sourceBlobEndpoint = $"http://127.0.0.1:10000/{sourceAccountName}";

        string destinationAccountName = "account1";
        string destinationAccountKey = "Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==";
        string destinationBlobEndpoint = $"http://127.0.0.1:10000/{destinationAccountName}";

        string sourceConnectionString = $"DefaultEndpointsProtocol=http;AccountName={sourceAccountName};AccountKey={sourceAccountKey};BlobEndpoint={sourceBlobEndpoint}";
        string destinationConnectionString = $"DefaultEndpointsProtocol=http;AccountName={destinationAccountName};AccountKey={destinationAccountKey};BlobEndpoint={destinationBlobEndpoint}";
        
        string sourceContainerName = "mycontainer";
        string sourceBlobName = "seed.txt";
        string destinationContainerName = "testcontainer";
        string destinationBlobName = "sample.txt";

        try
        {
            BlobServiceClient sourceBlobServiceClient = new BlobServiceClient(sourceConnectionString);
            BlobServiceClient destinationBlobServiceClient = new BlobServiceClient(destinationConnectionString);

            var sourceBlobClient = sourceBlobServiceClient.GetBlobContainerClient(sourceContainerName).GetBlobClient(sourceBlobName);
            var destinationBlobClient = destinationBlobServiceClient.GetBlobContainerClient(destinationContainerName).GetBlobClient(destinationBlobName);
            
            using (MemoryStream ms = new MemoryStream())
            {
                await sourceBlobClient.DownloadToAsync(ms);
                ms.Position = 0; // Reset stream position
                await destinationBlobClient.UploadAsync(ms, overwrite: true);
            }

            Console.WriteLine($"Blob '{sourceBlobName}' copied from '{sourceContainerName}' to '{destinationContainerName}'.");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
    }
}

Output:

Blob 'seed.txt' copied from 'mycontainer' to 'testcontainer'.

Reference:
azure - using multiple storage accounts locally using azurite at the same time - Stack Overflow by me.