Cheatin’ uh? Error

I went into a site I built a few years back (but which has been keep up-to-date plugin-wise) to make some changes to the ACF custom fields area. However, when I clicked on it, I got a weird error — this “Cheatin’, uh?” message: https://www.agentwp.com/fix-cheatin-uh-error-in-wordpress

I’ve actually never seen this before. I disabled the User Role Editor and it goes away. The odd thing is that I am logged in as the administrator — so I would think I have all rights, right?

Any thoughts on why this would be and if there’s a way to get around it?

Thanks.