The modal verb here is in the past, so I translated "You could have said no" which was accepted. Surprisingly, duolingo says: "Another correct solution: You could say no." This would be a statement about the present, not the past, and it states something that is somehow 'not real' ('could', not 'can'). Do we not need a particle like the Russian бы in these cases?
"You could have said no" sounds like "you had the possibility to say no, but you didn't".
"You could say no" can be hypothetical (Ви могли би сказати ні), but also could mean "you had the possibility to say no (and maybe you did)". Can't it mean that? "Great, you could say no and you said it, good job!"