One more common cause:
if you performed your client installation without activating the robot as service component, the Orchestrator is not able to start a robot on that machine if there is no active user session. This would explain why running with assistant works (so there is an authorized connection, folder access etc.) but not standalone.
To solve this, run your studio installer again, modify it by adding the robot service option. No worries, all of your other settings are maintained.
After the instalation sign off the user on the client and the machine should be available in the orchestrator for a run.