How do you offer a player in exchange when making a transfer bid?

I get the drop-down that allows me to offer players that re transfer listed but can't add the 'Other Player in Squad'?

Also, making an offer for Moussa Dembele, in he January window, was going to offer to let him stay at Celtic on loan, but that option is grayed out as well.

I am playing my AC Milan save.

Any help much appreciated.