Any suggestions? It’s a tough one…
When you apply for a developer account with your @handle, we send our verification email to the email address that you have associated with that @handle.
If you no longer have access to this email address, please go into your Twitter settings and update your email address. Then, you can go to https://developer.twitter.com/en/verify to resend your verification email address to the new account.
Let me know how this goes for you.