Is it giving you an error trying to do this or any kind of message? The player has to be willing to be traded to the team as well so it could be a case that the player will not do the sign and trade because he does not want to be traded to the team you are trying to send him to
Just adding here that I’ve got the same issue. Trade is accepted but just gets stuck. The sign and trade player has the new contract, but is listed as a free agent on their profile, but is with their old team still in the FA list.
I've raised a ticket with a link to some game files that highlights a couple of issues I'm noticing with sign and trades (they don't always complete, even in commissioner mode, and players who were ever a S&T option, will forever appear as S&T options, every season afterwards during free agency)