I've been googling for an hour and can't find it again. I might have the civilization wrong but I'm pretty sure it was the Inca. IIRC, one of the characters was a princess named Sumac.
IIRC, the story is about a general who rebels against the king. Princess Sumac is in love with the general. But the king doesn't know Sumac is his daughter. In the end, the general becomes king and goes and finds Sumac.
I believe the play you are looking for is called Ollantay. The play itself might have been written later than the dates you give in your question, according to The Cambridge History of Latin American Literature:
Although based on legendary material with roots in pre-Conquest history, the play seems to respond to the circumstances of the [rebellion of 1780].
Ollantay is the name of the Incan warrior in the story and he indeed falls in love with a princess, Cusi Ccoilor. They have a daughter named Yma Sumac.