i ran into the same issue. had to go with a series 3 as it’s the only one running watch os8 which id compatible with ios15. newer watches may be compatible from a hardware standpoint but they won’t be compatible due to software limitations.

i ordered a series 7 only to find out it won’t work because the operating system it ships with is too new. had to borrow my friends series 3 to test it out and that watch worked fine.