I got Ovid as a great person. The game tells me that Ovid "must occupy a district or wonder with a free slot of the proper Great Work type".
What are those?
Ovid is a Great Writer, which means he produces works of writing.
The simplest solution is to send him to your capital, as the Palace has 1 slot for any kind of great work. If your palace is full, your next best bet is to build an Amphitheatre in a Theatre District, and activate him there. (Amphitheatres have 2 slots for Great Writings).
Beyond that, some wonders also have slots for Great Works of Writing: Bolshoi Theatre, Broadway, The Great Library, and Oxford University.