Couldn't assign Critical Permission set to Selected User

Q: I got an error message 'Something went wrong! Couldn't assign a critical permission set to selected User'

A: This error message may appear if we are not able to assign 'Formstack permission set' to the intended user for one or more reasons.

Please try assigning the 'Formstack permission set' to the user you want a license for in Salesforce default permission set assignment page. This can be found in Salesforce under Setup > Permission Sets

Once you have added that permission set successfully, then go back to Formstack Admin Panel and try adding that user again.