App Permissions

Under Administration > Global Permissions, 'can use' must be selected for the specific app for the specific user or a group in which a user is included
1. See Atlassian's Adding or Removing Users in Groups documentation for how to view/modify a users groups

Additional details can be found via Atlassian's Global Permissions documentation

Space Permissions

Navigate to the Space the user was working in when received error
Under Space Settings > Permissions, verify the user or a group the user is included in has the ability to 'View' and 'Add'
Under Space Settings > Permissions > Add-on, verify the user has the ability to 'View' and 'Add'

Additional details can be found via Atlassian's Space Permissions documentation

Page Permissions

Navigate to the page where the user received the error
View page restrictions
1. Click on the lock icon in upper left corner of page OR
2. Click on the ellipses > Restrictions
Ensure user or a group the user is included in is listed in the page restrictions and/or, if applicable, the inherited view restrictions

Additional details can be found via Atlassian's Page Permissions documentation

Page has restrictions, none inherited

Page has inherited restrictions

Page has restrictions including inherited

Atlassian Documentation

The following links go to Atlassian's Cloud documentation. If you're using Server be sure to choose your server version in the upper right corner of their documentation as the user interface/navigation varies among versions.