@Phil_79 Hi and welcome to the Forum.
A few questions to start:
Did you check the SHA256 checksum for the downloaded Zorin 15.3 Core iso before creating your installation media?
Are you installing from DVD or USB, also can you tell us the exact error message you get?
If from USB, which bootable media creation tool did you use?
Are you just installing Zorin on its own or dual-boot with Windows?
Sometimes the problem is associated with a particular USB pen drive, even if this works in "Try Zorin mode".