could have

No, you can't say that. If you say "I could have gone to university", you didn't go there.

so what am i supposed to say ?
i was able to ?
isn't there any modals to express it ?

I'm not sure what you are trying to say. Why do you want to express that you were able to do it when you tell the reader that you did it anyway in the same sentence? You could say "I had the chance to" or "I had the opportunity to", but that may not be what you want.

hmmm...
for example
someone was able to go to university at age of 15. and i don't know if he attended university or not.
by using " had the chance " meaning is not changed but the way of conveying the meaning is changed though.

i want to use a modal to express an ability in pas.
could i used for general ability, and i think it wouldn't be used in this context.

You can say "he could have gone to university when he was 15" when he didn't do it. But it's kind of strange when you say "I / he could have gone to university" when you/he actually did it. "Could have done" means that somebody was able to do something, but he didn't do it for whatever reason. To me, this only makes sense when you say "he had the chance to, and he did it".

i read about it that by using that structure it most often means the action has not been done, but it didn't said 'always'.
i'm not a native speaker and i cant understand how it sounds when somebody says it to a native speaker

This is called "unreal conditional". "Could have" implies that the action you describe did not happen in reality. So by definition, you can only use this expression to describe things that actually didn't happen.

But I think I now understand what you mean. In my native language (not English) there is a way to say this using the equivalent to "could". I think this is not possible in Englis

navidresomeone was able to go to university at age of 15. and i don't know if he attended university or not.

He could have gone to university at age 15.

Two meanings:

1. He had the opportunity to do so at age 15. If he had wanted to, he could have done that.
2. He might have done so, but I never found out if he did, so I don't know if

so the fist one means he didn't go to university ?

Yes. The first one means he didn't go, the second one means that he could have gone, but you don't know.