I am doing an express check transaction and when I log in to my Sandbox account, it gives me an error:
This Sandbox email address is not available. Enter a different email address
Currently, Paypal has launched a new API. Only the beta version of the SDK is also available. Sandbox is currently unavailable, so you cannot access or verify its use.
I think that paypal will provide you with the correct error message, the sandbox will only allow you to log in with the email address that is associated with your email with the mailbox.
Make sure the address in the address bar reflects the URL of the sandbox, not the real-time URL. You can also try clearing your cookies and cache before opening a browser and entering developer.paypal.com. There were some problems that caused problems with registering and clearing cookies, and the cache fixed some of them.