Hi there,
Unfortunately, only Serbian citizens are able to buy agricultural land across the country. You'll need to set up a company and register the land in the name of that entity. Please bear in mind that there are extra charges (VAT) for transfers of property in the name of a company.
Serbian residents (permit holders) are allowed to purchase residential real estate if their country of origin has a reciprocity agreement with Serbia (i.e. Serbians are allowed to buy residential real estate in those countries). You will not be able to register the property in your personal name unless you become a Serbian citizen (which normally takes 3 years of residency before you're entitled to apply for citizenship).