Symptoms

I'm getting "To design K2 applications you must be a member of a group that has K2 Solution Designer permissions."

When running registration wizard for this site, the wizard complete with the following “Warning: This application has limited rights when installed from the SharePoint Store. Click here to learn more about the limitations and how to enable full functionality.”
 

Diagnoses

The issue was caused by K2 for SharePoint App has not been trusted properly.
 

Resolution

Navigate to Site Content > K2 blackpearl ellipses > ellipses > Permissions > click “here” to trust it again. Then rerun Registration Wizard.