This sounds something like the problem reported here:
Apple watches cannot be updated to watchO… - Apple Community
In that thread, the OP's watch could not be updated to 26.6 from 26.3 Watch OS when the paired iPhone was on iOS 27. Some people who had experienced this did post their solutions some of which apparently worked, but they did not work for all who had reported the problem, so there may be different wrinkles to this.
[My own experience -- my watch was on 26.6 and iPhone was on 27.0.1 and my watch then updated to 27.0.1 no problem, but the issue happens for those on a Watch OS earlier than 26.6, so my update went smoothly.]
From other non-Apple sites that frequently write about Apple hardware and software, there were comments when 26.6 came out for the watch that this update was to make the watch as ready as possible for the upcoming OS 27. But I can't find anywhere any indication from Apple that the watch must be on 26.6 and the iPhone on 27.0.1 for the watch to update to 27.0.1, nor can I find anything that states that users must update to Watch OS 26.6 before the iPhone and Watch go to OS 27. But for the people with this problem, this seems to be the case, e.g. if the watch is on something earlier than 26.6 when the iPhone goes to iOS 27, some users are saying they cannot update their watch at all anymore, to anything.
Definitely read through that link I provide above. Some folks seem to have found ways to proceed, hopefully what worked for them will also work for you.
