There are some civs (and maybe religions or civic combinations) that can build unit-producing buildings but cannot build the units those buildings produce. I don't remember all the cases, and I some might have already been fixed in 0.13. The most common cases involve the grove and golem workshop. I was playing Luchuirp with Fellowship of Leaves and I could build groves but could not build druids. In 0.12, I recall playing the elves and being able to build golem workshops, but I've played them again recently and I think that's been fixed in 0.13 (?).