I'm making a spell that targets any Overlard on the map and converts it to an Overseer. I gave it 500 range and set up everything in what appears to be the correct way. Then, I test the game and it says "Must target unit" when I cast it on an Overlard. ???

Oh, while I'm at it, how to make the ability only target Overlards (like Calldown Supplies only targets Supply Deposits)?

Make sure the filters allow overlords (Air is selected, ground is excluded) and also only allow player (enemy, neutral, and ally are excluded). To make it only select overlords you have to create or re-use a validator. The Validator should be "Unit-Filter" and makes sure the target is an overlord.

Yah, did all that. Now it says "Must target air units" instead of "Must target units". Otherwise, no change.

I'm clicking RIGHT ON the Overlard... Why won't the gosh-darned spell cast?

Mind posting the map?

What type of ability are you using? I would imagine Effect - Target?