I wanted to know if the term “Nail Spa” or “Nails Spa” is correct? I am opening a nail salon and would like to know which one would be correct. I know that Nail salon is correct not nails salon but referring to spa seems different to me and I cannot find an answer online about nail(s) with SPA
It's nail spa for the same reason it is nail salon, grocery store, car dealership, book store, pastry shop, etc (instead of groceries store, cars dealership, books store, pastries shop).
When a noun is used as an adjective, it is (almost) always used in the singular, including words that don't otherwise see singular usage:
trousers, trouser press
pyjamas, pyjama top
"Nail" is no exception:
nail, nails, nail spa