Hello,
I can’t turn this quest.
Thenios isn’t standing the spot he should
Is this quest important for the main campaing in Shadowlands?
I suggest you read up about this quest on wowhead.
There are a number of possible solutions mentioned in the comments section, too many for me to copy/paste here.
Plus an explanation as to why it happens, and one acknowledgement that in a specific situation, it is permanently bugged, until Blizzard devs fix the Kyrian phasing issue (if you complete the Kyrian campaign before turning this quest in, it enters a different phase, with no way to get back to the ‘correct’ phasing to turn it in, apparently).