What does 拈得せん mean there? Is some classical construction? I cant quite find an explanation for it
As you said, it's from a Buddhist (zen) verse 証道歌 which is attributed to 玄覚. As most of zen works are, it's written in colloquial Middle Chinese, not using the usual vocabulary of Classical Chinese or Japanese.
question + か + verb + む is a formula in Classical Japanese to make a rhetorical question, 爭【いか】でか拈得せん would be translated "how could (you) pick it up? (no you couldn't)".
(爭 is also a Middle Chinese word otherwise shouldn't have such reading.)
This sentence seems to be from this kanbun Buddhist sutra. 拈得す seems to be a rare word, but from its kanji (拈 = 撚 = pinch/pick/take; 得 = obtain), I think it means something like "to pick out" or "to pick and obtain". せん is a form of す ("to do") followed by ん for inference. This せん translates to するだろう(か) in modern Japanese.
Catching the moon in the water...how can you pick it out? (i.e., no it's impossible)