租不起：(you )can't afford the rent (coz its too high or you just don't have enough money) 租不了： (you )can't rent this apartment (can be any reasons, e.g. you can't afford it, or the owner of the property simply don't wanna rent it to you because of some reasons)

租不起 vs 租不了 (Not the same meaning) 租不起: Can't afford it. (Because the rental is too high) 租不了: Don't want to rent it. (Because of any reason)

租不起 is one of the reasons of 租不了。 租不起 means we haven't enough money to rent it。 租不了 means we can not rent it but the reason is not clear. (Maybe it's bacause the shop is closed/ it has been rented/ we haven't enough money.)