Hello @Gary Longford
I understand that you are trying to upload your RSA key to Azure Key Vault so it could be used in Azure Data Factory and you are getting an error while testing the connection:
When you are trying to upload the RSA key, the web browser experience to copy the key within a secret. By doing this, the format of the string changes from Base-64 encoding to string.
Try to upload the RSA key using PowerShell:
#Upload RSA key
$PrivateKey = [System.IO.File]::ReadAllBytes("C:\Your file")
# Convert to base 64 and to secure string
$Base64 = [System.Convert]::ToBase64String($PrivateKey)
$Secret = ConvertTo-SecureString -String $Base64 -AsPlainText -Force
# Upload key to Azure Key Vault secret
Set-AzureKeyVaultSecret -VaultName 'KeyVault-Name' -Name 'SecretName' -SecretValue $Secret
https://www.techtalkcorner.com/upload-ssh-key-azure-key-vault/