I’m rebuilding a a Suzuki LT 125 and it won’t turn over when I put the head/valve cover on. Could it just because the valves are so out of adjustment? Engine Turns over fine with cover off.

 

Valves are both closed if you have the cam set at the timing mark before refitting the head, if you haven't done that, and fitted the head with a valve open, you may have bent a valve, so check this before fitting the head again. You could have got a faulty piston which is hitting the head at TDC. Or the head skimmed too much so the piston hits it.

The engine will get hard to turn with the head on if the plug is in, because of compression and again, slightly harder to turn over once the cam chain is on, as the chain has to pull the valves open against the springs.