Has the apartment been made unlawful due to sins, or is the provision that a person is deprived of due to sin one of the calamities decreed for him? And if he had not committed the sin, would it have been possible for him to acquire the apartment?

The effect of sins on sustenance is affirmed by the Sunnah, and a servant may be deprived of their sustenance due to a sin they commit. It cannot be definitively stated that the difficulty in purchasing the apartment is due to sins; it could be for another reason known only to Allah, or a result of Istikhara (seeking guidance from Allah), as what the one performing Istikhara wishes for is not necessarily what is best for them. You must be content with what Allah has allotted and repent from your sins.