We have been researching this issue and suspect that there may be an interaction between AppArmor and Samba which is causing this issue on some systems with Zorin OS 18.
In order to resolve this issue, please try opening the Terminal (Ctrl+Alt+T) and entering this command:
sudo apt remove apparmor-profiles
Afterwards, please restart your computer to apply the workaround. You should now be able to use Samba normally.
Please let us know if this workaround resolves the issue for you. This would allow us to apply a more permanent solution for all Zorin OS 18 users.