User Can't Access Confluence Cloud
A user or more are unable to access the Confluence application in Cloud, but they have the ability to access JIRA and other apps within the same instance.
Cause / Resolution
There's few possibilities that can cause a user to not be able to access Confluence :
1 - First thing to check always is the application access within your instance and ensure the user has access to Confluence.
2- In some cases this can be related to the Global permissions for Confluence, if the user does not have view permissions he/she won't be able to view any Confluence space.
By ensuring the above settings are in place, the user in question should be able to access Confluence without much trouble.