Learn English – Difference between “Why couldn’t you” and “Why couldn’t you have”

That's just the structure of the exercise. Where the blank is, it isn't grammatically correct to say "could". But if the exercise allowed you to change the sentences a bit, you could very well use could.

In the first sentence, the correct syntax is used to + verbal root, so you have to put be able to. You wouldn't say:

26.1.4 I used to could stand on my head, but I can't do it now

But you could say:

26.1.4 When I was young, I could stand on my head, but I can't do it now.

Which is grammatically correct.

Same story for the second one:

26.1.5 I can't understand Martin. I've never could understand him.

Isn't grammatically correct. I've never requires a past participle after that. If you had the ability to modify the sentence, you could say:

26.1.5 I can't understand Martin. Actually, I never could understand him.

Generally we use "was/were able to" or "managed to" instead of "could" for past archivements in affirmative sentences when we are talking about facts and not possibilities (Cambridge dictionary). This is because "could" can be ambiguos and make us guess whether it was an ability or a possibility. Here's an example:

• She could fix her computer.

This sentence may either mean "She could fix her computer but she didn't" or "She was able to fix her computer"

Most sources state that we shouldn't use "could" in this case if we are speaking about a past archivement. If we wish to speak about possibility in the past we should use "could have + -ed form of a verb":

• She could have fixed her computer.

This undoubtely means that she had the possibility to do it but who knows whether she did.

Notice that in negative sentences both wasn't/weren't able to and couldn't are possible for past archivements.

• "I wasn't able to/couldn't come because of the bad weather"