I do not understand this statement. Why would you need to install Ubuntu?
If you installed and made changes on Ubuntu, that would affect the Ubuntu install, not the Zorin OS install.
Please boot Zorin OS, but access the Recovery Menu instead of proceeding to a full normal boot.
In the recovery menu, arrow key down to Root - Drop to Prompt.
Hit the enter key
In that terminal window, run the commands to change Grub File adding the nomodeset parameter:
Alternatively, you can try testing the new Zorin OS 17 Beta now available instead of installing Zorin OS 16.3: