Yes, same has happened to me once or twice. I already had a SQL Server standard installed, but NAV still installed an SQL Server Express instance in a newer version. So check for a different SQL instance. And always “RUN AS ADMINISTRATOR”, whenever you need to install NAV. Even if your account is setup as the administrator, then this is still required for the installation program to gain full access to your computer.
You may also just select to replace existing database when installing. This is one of the custom options you can change, before installing.
And as a separate question. If you need a NAV 2018 demo to “play with”, then why didn’t you use Docker? Personally I have not even installed NAV the last year or so by running SETUP.EXE. For demo, test and development purposes, then I only use Docker containers. And I do install NAV a lot! With Docker, then all you need to install any version of NAV, from NAV 2015 and up till the newest previews, is a single PowerShell command and NavContainerHelpers. Learning to install Docker and use it with NAV takes an hour or three. There are plenty of documentation on how to do it.
I have moved your question from the developer forum, into the technical forum. This is where this question should have been asked.