You don’t have to show the apartment if you don’t want to.
Again it depends what the lease says. It does seem to me the landlord has no right to come unannouced and show an occupied apartment on any day of the week let alone Shabbos. However there might be some clause that contracdicts that and allows the landlord to call and show up (Maybe he called 2 hours before on Shabbos and the OP did not answer the phone)
ALWAYS read a lease before signing one and if you cant understand the legalize , get someone who CAN read it and explain it to you