What was found
PILOT REPORTED THAT WHILE ON TAXI OFF THE BLOCK THAT AFTER LIFTING THE LEFT HAND POWER LEVER INTO GROUND FINE, IT BECAME STUCK. THE LEFT POWER LEVER WOULD NOT MOVE AND NEITHER WOULD THE RIGHT POWER LEVER. AIRCRAFT RETURNED TO PARKING BLOCK AND ENGINE WERE SHUT DOWN WITH NO FURTHER DISCREPANCIES. MAINTENANCE FOUND DURING TROUBLESHOOTING THAT THE POWER LEVER DETENT SUPPORT BLOCK WAS MISSING A SCREW THAT HELD IT IN PLACE ON THE CENTER COLUMN AND CAUSED THE THROTTLE GUIDE PINS TO BIND. RESET THROTTLES AND INSTALLED NEW SCREW AND ENGINE OPERATIONAL CHECKS WERE SUCCESSFUL.