According to this site
When we talk about ability, we mean two things.
First, we mean general ability. This is something that once you have learned you can do any time you want, like being able to read or swim or speak a language, for example.
The other kind of ability is specific ability. This mean something that you can or can't do in one particular situation. For example, being able to lift something heavy, or find somewhere you are looking for.
could / couldn't (for general ability)
I could read when I was four.
My grandfather couldn't swim.
was able to / couldn't (for specific ability)
When the computer crashed yesterday, I was able to fix it.(not 'I could fix it')
I couldn't open the window.
The above explanation means "We have to use "Was able to" to express a specific ability in the past in positive sentences"
My question is that "is it ok to use "Could" to express a specific ability in the past in positive sentences"
Is it ok to say "When the computer crashed yesterday, I could fix it."
Or we must say "When the computer crashed yesterday, I was able to fix it."