You'll need to make sure that the group you want to post in has the PopItUp app installed before posting to Facebook. This is a new requirement from Facebook as of the beginning of August 2018.

Installing or reinstalling the app is the most common solution to the error: 'post' error: [200] (#200) Insufficient permission to create album

A video demo for the process is available here.

You can also read Facebook's Help article on the topic here.