The problem occurs because the TMCM service account is using a different account instead of the local system. Change the account to the local system account and restart the services to fix the issue:

1. Go to Services of the Microsoft Management Console (MMC).  
2. Double-click the TMCM service from the list.
3. Select Log On > Local System account > OK from the pop-up window.
4. Manually restart the TM service by going back to Services, right-click the TMCM service and select Restart.
5. Log on to TMCM and click Administration > Account Management > User Account and the Import AD users option should appear.