You have Microsoft® 365™, which may or may not be federated to RM Unify. Your users are licensed for Microsoft 365 and have the Microsoft Teams app enabled within that licence plan. Within 'Microsoft 365 admin center', Settings, Org settings, Microsoft Teams, your users have the 'Turn on Teams for users with the following licenses' box ticked.

However, when your users open the Microsoft Teams desktop app and enter their credentials to sign in, they are presented with an error message, e.g.:

"Request Id:  2dc2b049-8a69-4a0c-a4ce-b74a96ee7800 
Correlation Id:  654e9049-1402-4fec-81a2-c97f9b4e2617 
Timestamp:  2021-03-30T16:28:35Z 
Message:  AADSTS7000112: Application '1fec8e78-bce4-6aaf-ab1b-5451cc387264'(Microsoft Teams) is disabled. "

Sign-in to the Microsoft Teams enterprise application has been disabled in the 'Azure Active Directory admin center'. 

1. Log on as a Microsoft 365 global admin and browse to the 'Azure Active Directory Admin Center': https://aad.portal.azure.com/<your_365_domain>.
2. From the left hand pane, select 'Enterprise applications'.
3. Under Manage, select 'All applications'.
4. Change the 'Application type' filter to Microsoft Applications and the 'Applications status' to Disabled, then select Apply.
5. If the Microsoft Teams application is returned, select it and under Manage, select Properties.
6. Change the 'Enabled for users to sign-in?' setting to Yes and select Save.