Error Detection in sentence

No. During is wrong. It cannot be followed by a subject (I) and a main verb. He's always been = He has always been.

CB

SANGEETA MAHESHWARIDuring I've known him, he's always A B C been kind and generous. D (A) During (B) I've (C) always (D) been Error is been.

As sir CB has said, the error is "during",  and  not  "been". Your sentence should be like this:

Since I've (I have) known him, he's (he has) always been kind and generous.

Am I right, sir, using

I wouldn't use since at all as it often means 'because' as a conjunction. It doesn't sound good with have known in your sentence. I would say He has been kind and generous for as long as I have known him.

CB

The potential ambiguity of Since could be avoided by saying Ever since I've known him...

Cool BreezeI wouldn't use since at all as it often means 'because' as a conjunction. It doesn't sound good with have known in your sentence. I would say He has been kind and generous for as long as I have known him.

Thank you very much, sir, for guiding me.

So, is it wrong to say?
"I have liked her since I have known her"

No, they are not completely wrong, although both are a little awkward. However, as since can mean either from that time on, or because, there is the possibility of ambiguity, so it is best to use a construction such as ever since or since then in any sentence where since could be mistakenly interpreted as because.