What I did and worked
1. Uninstall the software in the computer that you want to remote access and could not due to the: Service Key request failed.
2. Install again MSP360 Remote Assistant
2. Click in the OFF of the Unattended Access
3. Before changing this option to Yes, you have to give FIRST: Allow Full Access (you will find it in the second option of the settings in the left menu)
4. Click OK to Full Access
5. Go back to Unattended Access and Click to allow it…. then provide a password to use it when remote unattended controlling.