I used to buy players from my same friends just 1-2 months back
hmm, ok let's start from the basics
- you have enough token and money
- the other team has at least 15 players with contract
- this player better be in subs and not main squad
- he isn't loan in another FA group team

what else ?
do the classic things uninstall- install, clean cache

and one possibility (but really rare), the other team or yours removed to a different server