Sometimes even when the app is successfully installed on a new Confluence Cloud instance the users may get this error:
This happens due to a known Atlassian issue when the app user does not belong to any group that has permissions to access Confluence:
confluence-users
In this case, the Confluence Admin on the customer end will need to grant access to these groups to the app user. They should be able to do this via REST API, alternatively, they can open a ticket for Atlassian Confluence Cloud Support asking to add these users to the groups above.
App user information to open the support ticket at Atlassian:
App User Email: cloud-files-google-drive-docs-office-365-documents-dropbox-topdox-for-confluence@connect.atlassian.com AAID: 557058:6baf98fa-dc04-4208-894a-6e9a902a4611
You can find more information about this known issue on the links below:
https://community.developer.atlassian.com/t/add-on-user-permissions-problem/48760