The chain is apparently not running on the sprocket. Loosen off the adjuster fully, you should be able to push the roller back with a screwdriver from above through the chain tunnel then tighten the clamping screw.
Now just let some slack chain down into the crank area and move the chain back and forth and pull up to seat the chain on the sprocket... trial and error, I fear.
Removing the sump should let you see the sprocket, I don't know if you could touch it though.