Settings to enable the store button are in Onyx settings -> header -> navbar

If those don't work:

Verify the Link

If you are using the Tebex, Craftingstore, MC Trade, or the store module, make sure the store link in onyx settings is /store/

External Stores

If you are using an external store, make sure you create a redirect link to it in StaffCP -> custom pages. Then, make sure the store link in onyx settings is set to the link of the external store, NOT the link's url on your site.