Hi, I don’t have any content bought, but in the campaign I’m in, we have some, and it shows that half-elf’s have subraces, but when I try to make one, it doesn’t show that as an option. Am I doing something wrong?

There can be no subraces for the standard Half-Elf. One of two things must be happening, either:

1. Those “subraces” are actually “variant races,” which are possible. In fact there are several official variants to the half-elf, which is what I suspect you were most likely seeing, but they might have been homebrew.
        Or:
2. Someone made a homebrewed copy of the half-elf and enabled subraces so they could make them for that homebrewed half-elf.