This article is for Cloud.
We have customers who got this issue resolved with the help of Atlassian Support. The message from Atlassian was as follows:
I found that the app can't be install due to it can't gain access to your site. The most common cause of this problem is that you don't have a default access group where the app and other users are added to gain access to your products.
If this is the case, please check if any of your Atlassian applications has a default access group configured. Please follow this Atlassian documentation: https://confluence.atlassian.com/adminjiraserver/manage-group-access-to-applications-938847042.html or this Atlassian Community post: https://community.atlassian.com/t5/Jira-questions/How-do-I-edit-the-Default-access-group-in-JIRA/qaq-p/1232781
For further assistance on configuring the default access group, please contact Atlassian Support at https://support.atlassian.com/contact/