1. I’ve been trying to reach her for a week now, but I still can’t - Correct. 2. I’ve been trying to reach her for a week now, but I’ve been able to yet - Wrong. Contradictory statement. 3. I have been trying to reach her for a week now, but I haven’t been able to do it yet - Confusing. 4. I’ve been trying to reach her for a week now, but I haven’t yet”? - Confusing. (2) doesn't make any sense to me. The use of "yet" is wrong and I can't guess what the sentence is trying to say. (3) and (4) are unclear because "trying to reach her" sounds like you have been attempting, but the part after "but" seems to say you have not actually tried. Here are some other ways to say it. - I have been trying to reach her for a week now without success / with no luck/success. - I have been trying to reach her for a week now, but it seems impossible.

1) "I’ve been trying to reach her for a week now, but I still can’t" ok, but sounds awkward. I wouldn't say this myself. 2) "I’ve been trying to reach her for a week now, but I’ve been able to yet" Incorrect. It does not make sense. 3) "I have been trying to reach her for a week now, but I haven’t been able to do it yet" This is good. 4) "I’ve been trying to reach her for a week now, but I haven’t yet” This is ok, but somewhat awkward again. I would use #3.

Thank you, Gareth! What about “I’ve been trying to reach her for a week now, but I haven’t yet”?

None of those sentences is really bad but some sound more natural. I think most people in everyday colloquial speech would say either "I’ve been trying to reach her for a week now, but I still can''t" or "I’ve been trying to reach her for a week now, but I still haven't been able to’". “I have been trying to reach her for a week now, but I haven’t been able to do it yet” isn't wrong but I think most native speakers would not use the "to do it".