PHP is NOT allowed in your code (disabled by your theme or a plugin)

Question

I am using the Head, Footer, and Post injections plugin and it is displaying this error. I am using the plugin to create a lightbox that will handle booking appointments, and everything about the lightbox works great except I need PHP to send an email. I honestly have no idea how to even use PHP, but I found a guide on how to send emails with it. Anyways, I tried disabling all other plugins and changing my theme but that didn't get rid of this error.