"Valid excuse" means having a legitimate reason for not doing something or for doing something differently than expected.
Sentences with «valid excuse»
My daughter didn't take veggies to lunch every day and there are valid excuses for why she didn't. (thelunchtray.com)
-- Another sometimes valid excuse, it's more often a strange excuse often used by people who like to self - diagnose and have never actually been told this by a doctor. (leangainsguide.com)