If there are simply no buildings showing up in the build tab, it's because you've built everything for your settlement level, and have to wait till your population grows large enough to move to the next set of buildings (i.e. large enough to build a Governor's Palace, or a Proconsul's Palace).

If they're showing up but washed-out and unclickable, then you've spent all your cash already and will have to wait till you get more later.