DIAGNOSIS The microwave is detecting an obstruction or a failure in the turntable system. The most common cause is the roller guide (the little plastic ring the plate sits on) being misaligned or stuck.
FIX Unplug the microwave. Open the door and lift out the glass tray and the roller guide underneath. Check the guide for cracks and clear any debris in the channel it sits in. Make sure the guide's center hole aligns perfectly over the drive hub in the floor of the oven. Re-seat everything and plug it back in.
IF THAT DIDN'T WORK The issue could be the drive coupler (the white plastic piece under the floor that turns the roller guide). If it's stripped or broken, the turntable motor runs but nothing spins. You'd need to access and replace that coupler, which requires removing the outer case.
HEADS UP Never run it with the turntable removed or severely misaligned. The microwave uses that rotation to cook evenly, and running it stationary can damage the magnetron.