Experience Site Admin Cannot Log In As Experience Site User Even Though They Are a Member of the Site

This issue occurs when the Data Protection and Privacy setting is enabled in the org and the Experience Site Contact has an associated Individual record with 'Don't Process' or 'Forget this Individual' flags enabled, which prevents site membership from being established.

Check for the Individual Record:

A possible cause for this is the org has Data Protection and Privacy enabled, and the Experience Site Contact has an Individual record associated with it that is blocking the ability to make the site user a member of an Experience Site. 
 

1. Check the Contact in question.
2. Validate if the record has the Individual field present on the layout. 
3. If it does, confirm the field has an Individual record present. 
4. Edit the record and confirm if the record has the following fields enabled:
   • Don't Process
   • Forget this Individual
5. Uncheck these fields. 
6. Save.

Adding the User Back as a Member:

Once complete you will need to trigger the adding of the user as a member of the Experience Site. This can be done several ways. Three examples are:
 

• Disable the welcome email on the Experience Site and remove and re-add the profile of the user that needs to be a member. (This may not be the best course of action depending on the number of users that would lose access, and the time taken to refresh site membership).
• Create a dummy profile to temporarily assign to the user and then assign back the original profile. A profile change on a a user will trigger the membership update. 
• Deactivate the user from the contact and create a brand new user record. 

Once membership is re-established you can turn "Don't Process" and " Forget this Individual" back on and the site membership will remain intact.