I am sorry but if you may please take a look at that screenshot, just before the error, couple of lines ago, zPanel asks you to enter a full domain name.

But as per the screenshot, it seem you have entered only "u", which is not a domain name.

May be that is the issue.

Secondly, zPanel is dated and now have been moved to new project called Sentora, if you plan to use zPanel, its better to install the latest one, that is Sentora.

Thanks.