Engine runs fine but head will not spin - whipper snipper stihl fs 38

if it is a bent shaft then the cable is broken in the tube
if a straight shaft then a couple of possible items
the square drives on the end of the shaft is rounded out or the clutch is not expanding as required ( same for bent shaft)

Sound like low engine compression
head gasket gone between the cyl

ckeck thefan clutch if you can spin it easy by hand eng. off! make sure the air is bled out of the cooling system if the fan is easy to turn its junk

Are the drive shafts engaged properly? you need to remove all the plastic engine covers and handle to access the clutch. I can send you the IPL if you need it.

check the cable that runs through the shaft from engine to head, it may be brocken or striped but when theres no pressure on the head it spins ok

The arbor is twisted into

it sounds like the clutch may have oil on the shoes or oil in the clutch drum if it has a clutch or the clutch is binding and not fully locking up.

The shaft cable has either come loose at the head, or is broken at the engine most likely. Is this a 2 piece unit? If so, see if the cable turns with the grass trimmer section removed. If so, it is usually easiest to just replace the trimmer shaft section. One piece units require disassembly at the engine, and if you are not mechanically inclined, that can be a pain.

rotate attatchment head slightly while inserting driveshaft

sounds like the drive cable is broken inside the shaft some flex drive cables can cost half to three quarters the price of a new machine.