Hello,
Zorin OS is designed to boot in Legacy mode normally without any additional setup. I would recommend you to look at the Boot options in your computer's BIOS settings screen to make sure that Legacy mode is enabled and the other boot settings are in order. Please also note that you must have booted the Zorin OS Installation USB in Legacy mode for it to correctly install in Legacy mode and boot afterwards.
If the boot settings look fine, there may be an issue with your installation medium. We would recommend you to check if the installation medium may have defects by following the steps on this guide:
https://zorinos.com/help/install-zorin- ... tall-driveIf defects are found, we would recommend you to try burning the downloaded Zorin OS .iso file to a different USB drive, as the first drive you used may have experienced wear & tear from prior use or had other physical defects.
If the issue still persists, it might have been caused by a corrupted download file. We would recommend you to try downloading the Zorin OS .iso file again using the latest version of Firefox or Google Chrome without any third-party download managers (in order to avoid file corruption). You can verify whether the file has been downloaded correctly without corruption by verifying that its sha256 checksum matches the one listed after pressing the "View sha256 checksums" button on this page:
https://zorinos.com/help/install-zorin- ... d-zorin-osTo verify the checksum, we would recommend you to use an app called QuickHash, which can be downloaded from here:
https://quickhash-gui.org/downloads/After opening QuickHash, please navigate to the "Files" tab and select the "sha256" Hashing Algorithm before clicking the "Select File" button to choose the Zorin OS .iso file and waiting for the checksum to be generated.