Plaid requires that you have a valid phone number associated with your Plaid Portal account in order to receive a text message authentication code when you log in.
If your phone number has changed, and you're no longer able to receive this code at the phone number associated with your Plaid Portal account, you'll need to create a new Plaid Portal account.
If you still have access to your existing phone number and can log into your Plaid Portal account, then you can delete your account from the Settings page. This will allow you to then set up a new account with your new phone number.