Hi
@370155,
This is working as intended as The Sims 4 DLCs bought on Steam do not work with The Sims 4 on Mac, as the game on Mac is only available via the EA app for Mac (or Origin for Mac).
On the Steam page for The Sims 4, macOS is also not mentioned as one of the supported platforms:
https://store.steampowered.com/app/1222670/The_Sims_4/.
Hope this clarifies a few things.