Why is the verification code on my w-2 not valid?

If you're talking about the entry in box d, it's most likely because the provider of that W-2 has not yet made it available on line. Until they do (and that's assuming they do at all) it's not valid. You can just leave that box blank and press on to enter the W-2 manually.

The verification code means nothing ... you can delete it .... 

https://ttlc.intuit.com/questions/2928817-what-if-the-verification-code-box-on-my-w-2-is-empty