Hoping someone can help - I can't seem to find any articles anywhere to address this, so perhaps it's unique to me!

Whenever I add a horizontal rule from the editor I get an error message (attached) once it's published. It doesn't matter whether I use the editor or add the code manually the result is the same.

I'm a bit of a novice technically so it might be something simple but I'm really stuck! Every site I build in SS has the same problem.

Hoping someone can help me out!!

Cheers