This is likely an issue with either your Forms account, or your cached credentials (username/password) within Flow. Please log in to forms.microsoft.com using the credentials to insure that your account is operating normally.
If your Forms account is working when you log in, navigate to the Data > Connections page in Flow:
Then navigate to the Microsoft Forms connetor, and choose either Switch account to re-login with your credentials, and/or Delete the connection and re-add it, example: 
Once you've update your credentials in the connections page, insure that you are using the correct login under My connections on the Forms action, example:
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