Hello Dan. Welcome to our community 🙂
You'll have to disable RankMath's Schema module entirely to fix this.
From your WordPress dashboard sidebar, go to Rankmath » Dashboard
There you'll find a module named "Schema (Structured Data)" (or a similar name to that if they change it in a future update after I write this comment)
Simply disable the slider. Clear the cache and reload the dashboard.
Error should be gone by now. Cheers!