Cause:
This issue occurs when the action tag 'Internal IM (including allowed domains)' is disabled in Org Admin:
 


Solution:

The Org Admin would need to enable the policy to allow Internal IM:

1. Log in to the Org Admin console.
2. Click on the Policy Editor tab.
3. Select the policy from the list of organizational policies, then click the Internal IM (including allowed domains) action, and click Edit.
4. Choose Enabled, then click Save:
    
   
    
5. Next find the affected user under the User tab.
6. Click Edit.
7. Click on the Policy Group Assignment tab.
8. Add the policy again.
9. Click Apply, then Save to save the changes.