@SecretRiddle Hi and welcome to the Forum.
Couple of questions first to help us help you:
- Which flavour of Zorin are you trying to install, e.g. Core, Lite etc ?
- Have you confirmed the SHA256 checksum of the downloaded Zorin OS file is verified ? see this Methods of downloading Zorin OS
I have done a quick web search for the error posted in your title and found this, which may or may not help. Note Zorin 15.3 is based on Ubuntu 18.04, so fixes for that distro should be equally valid.