i recently sold a ship and was planning to use said ships captain on new ship didn't work i had too hire two captains to build another ship
IT MUST BE A BUG

Have you gotten any other ship meanwhile? The number of captains must be Current_ship_count+1 for it to work. For example: If you have 20 ships but 10 captains, you're gonna have to buy 11 captains to build another ship

i got 38 ships and 37 captains but i sold a ship it should have went down one

Ships gained from tidal waves or plunder come with a captain to keep them sailing : but that doesn't count towards your total officers ; that is fixed by how many you have employed and whether you have more or less ships, those paid-for officers stay with you. If you sold a ship which is 'extra' to your officer count, then that captain disappears, but as it was never counted the actual officer count would not change.

To build a new ship, you still need to have a spare captain, that is one more than the total number of ships you own. It's not a bug, just the side-effect of getting 'free' captains with plunders or TW's... sooner or later, you have to buy a 'proper' captain for it if you want to expand your fleets.