We use could for general ability. However, when you want to say that somebody did something in a specific situation, use was/ were be able to or managed to (not could). For example: There was a boy in the river but people were able to save him. This is not an ability, but a specific event. However, negative 'couldn't' is possible in all situations. I couldn't swim. We couldn't save the kid. He played well, but he couldn't beat me. Lastly, I will give you an example for you to think about it: Jack was an excellent player. He could beat anybody. (He had the general ability to beat anybody) Ted and Jack played tennis yesterday. Jack played great but Ted was able to beat him. (Ted, in this specific event, managed to beat Jack) I can understand the person who suggested 'repeated actions'. When it is a general ability, you will repeat it probably.

"Wasn't able to" tends to be a more formal way of saying "couldn't". You will hear more people say "Couldn't do or Jack Could not come today because of the kids". Sometimes you may hear "Wasn't able to" when something's a bit more serious...."Joe isn't (or is not) "wasn't" or "was not" able to because he broke his hand ✋ ". They are both Contractions of; Couldn't - Could Not Wasn't - Was Not Hope this helps.

You can't say couldn't able.