This means a battler has a non-numeric value for their HP. The default code has a safety check in place to prevent this error (non-numeric values become zero), but I'm guessing you're using Yanfly's Damage Core plugin, which removes that check.
The cause is likely an error in a skill formula: something doing non-numeric (e.g. undefined) damage. It could also be worth checking any scripted damages, i.e. anywhere you've used gainHp.
We are truly intellectuals.
Without using violence without weapons
Our group has as many as 1.3 million people and more than 1 hundred thousand talks.
We gathered in an hour, more than 4.5 Half a hundred thousand