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!"
On further reflection.
"He offered me a slice of chocolate cake and I just couldn't say no." however, that is not describing a single event.
The positive version is different.
- "He offered me a slice of chocolate cake and I could have said no." conditional
- "He offered me a slice of chocolate cake and I was able to say no." indicative
"He offered me a slice of chocolate cake and I could say no."
With other verbs it's different.
- "I could see the dawn." stative
- "I could speak Ukrainian." an ability that exited for a long time, but no longer exists.