In this code:

apply_constraints(MatchA, MatchB, PotentialMatches, RemainingMatches, Day) :-
    different_teams(MatchA, MatchB),
    can_play(MatchA, morning, Day),
    can_play(MatchB, morning, Day),
    subtract(PotentialMatches, [MatchA, MatchB], RemainingMatches).

the second can_play clause should be checking the afternoon rather than the morning. This would be more obvious if the games were called MorningMatch and AfternoonMatch rather than MatchA and MatchB.