I wasn't sure which forum I should post this on, so it's going here.

Does every band submission have to be of a band or can you submit a solo artist?

Yes, you can submit a solo artist. All we care about is whether there is an artist/moniker having a valid release publicly distributed under that name that is predominantly metal. Whether that artist is technically a band or just a single person doesn't matter. In fact, you will find many solo projects in the Archives.