I assume they are seeing this error in My Apps since that's usually where that error gets triggered. If that's the case you need to enable user assignment for the app to appear on the dashboard. Under Azure Active Directory > Enterprise Applications > Application > Properties turn the "User assignment required" toggle to "Yes."
Then create a group of users and assign those users to the application. Since you said you want ALL of the users to have access, you can create a dynamic group that assigns everyone with "accountEnabled -eq True" as a member.
From the Troubleshooting guide
"The application must be configured properly and assigned to the user or a group the user is a member of to see the application in My Apps."
If you want your users to be able to register applications in the portal and consent to applications managing company data, make sure those settings are also set to "Yes."