Account enable to partner for use site experience

Hi! I’m trying to convert a social account so that I can enable the Activate User option for its contacts, allowing them to use the Experience site.

However, when I click the standard Activate Account button for the contact, I get this error banner (shown in the screenshot), and I’m not sure why.

My user has admin permissions and the highest-level role. Has anyone else encountered this error or knows what might be causing it?