Learn English – ‘have not been able’ or ‘have been unable’

Don't be so hard on yourself. English is very difficult.

This talks about present perfect tense - which have been by itself falls under, and have been by itself can be used to describe an experience:

You can use the Present Perfect to describe your experience. It is like saying, "I have the experience of..." You can also use this tense to say that you have never had a certain experience. The Present Perfect is NOT used to describe a specific event.

Have been + {-ing form of verb} is present perfect continuous tense, which is different. That's used to talk about something that has started at some time in the past and is still ongoing or happening now.

Putting would in front of either of these now says that the action or experience did not happen because of something. Typically a clause will precede or follow explaining why, typically starting with but.

I would have been to Japan, but I was unable to afford the trip.

I hated her guts. I would have otherwise gone to the skating rink.

I would have been going with him, but he sort of creeped me out. So I went with her instead.

We would have ruled the entire kingdom, yet their relentless attacks on our land proved it impossible.

It's a very subtle difference. "Could not have" is used to definitively declare that an impossibility occurred in the past. As in, it was impossible for the person to be reached. It assumes the writer has full knowledge of the possibility of an activity. "Could not be" expresses that an attempt was made by the writer to reach the person, but it was unsuccessful.

An example is: "He could not have taken a call at 5pm because he was on a flight to Baltimore." The person declaring that would have to have that knowledge to know the impossibility. Another person who didn't have such knowledge would say, "I attempted to call him, but he could not be reached." They don't know why he couldn't be reached, or that it was impossible to do so.