If you are seeing this "can't create a new access token for user" error on your sending accounts, it means that your email provider is not able to share the token with Instantly or revoke the access of the token.
This usually happens if you changed the password for your sending account and did not update the account in Instantly.
The only solution for this error is to delete the account and add it again.
Click on the three dots and choose Remove account:
Then, go ahead and simply connect your account again.