@Richkm , Thanks for posting in Q&A.
Based on my test, to make the user have the ability to add the app, we can grant the permission under "Managed apps" and "Mobile apps" to the user group. Here is the permission I grant for your reference:
Then the user can add app.
Hope it can help.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.