I keep getting this error saying: 'You have too many open bids, your squad size will be over the limit'.

This is false though, since my 1st proper attempt was rejected (Thanks to this very error message), my actual open bids are ZERO.
I currently have 19 out of a limit of 27 players.

PLEASE HELP!

hey Vrikie, you are probably trying to bid a youth player, in this case the limit is smaller

Ah yes! That applies exactly. I see now that I've bid on 1st team players, it does work. I'll try and keep this in mind.