Thanks for confirming - from your screenshot, I can't see anything obvious as to why the validation could be failing so the only things I can suggest are:

1. Add the comment again
   If copying and pasting, double check for any extra spaces at the start or end and remove them.
2. Create a new INC and follow all of the steps again
   Annoying I know, but sometimes starting afresh is the best way.

If the validation is still failing and you're 100% certain that everything is as it should be, you'll need to raise a support case via ServiceNow University.