Why can’t I put spymaster in top left?

You made the mistake of putting a spymaster in front of your garrison instead of behind it. Or it destabilized?

Either way, gotta go back until that legion barracks is a garrison for you to be able to place a spy master. Can’t be touching another existing spy master. Sorry bud :(

You need to backtrack the last spymaster in the chain to place in the empty spot.