If you were neither redirected to a "Thank you" page with the license key after the purchase, nor have you received an email with it, please reach out.
First, please double-check that the key is correct, especially if you are not copy-and-pasting it.
If that doesn't help, make sure that your firewall, ISP or government isn't blocking access to one of these domains:
api.parsify.app (if using version 1.6.2 or newer)
api.gumroad.com (if using version 1.6.1 or older)
Finally, if the above solutions don't work, please reach out.