layout error
I´m not saying you can't do a single 1pF cap.
Probably the DRC error was based on how the cap was made (as soon as in one direction you have an overlap between metals below the required 10um, you'll have the error)
With my comment, what I was trying to say was that, if your process data makes that a 1pF cap need metal plates small enough to have a DRC error as mentioned (I don't think so, but...) you would need to use a bigger unit cell (for instance a 2pF cap) in order to avoid the DRC violation and create the 1pF one by using two parallel 2pF ones. Is it clear now?