it is exactly what the message says:
"Such a flange isnot possible as synchronous feature!"
A sync contourflange needs at least a small plane start and end surface and can not consist of a bended (cylindrical) face only.
So You either can add short linear extensions to the circle
or You switch over to Ordered, where such a contour flange is possible too!
no silly question at all, per definitionem a silly questions doesn't exist, only foolish answers!
Sorry for the late answer, but I was on phone the last half an hour.
And there are differences between sync and ordered in reagrd what is possible to do, and how it is possible.
But the Big Bang is, that You can use both methods together as You like and as You need.
I always preach to think about integrated modelling rather than synchronous modelling or ordered modelling.
Use both - thats the message!
Integrated is Sync plus Ordered!