nonce in custom form is not verifying

I have a shortcode which calls an html form:

The form loads on the WordPress page just fine. But when the form is submitted the nonce is not validating.

in the processing .php I’m able to print_r($_POST) and see the value of the nonce and other fields, but my condition won’t satisfy.:

I’ve done this many times, but never using a shortcode. Is that the issue?

Update:

As it turns out, it had nothing to do with nounces, but all WordPress functions were “undefined.” Once I added the following to the top of my script, everything worked.