Make sure you download the full version.
There is no demo mode in the full version, there are two completely separate versions of the simulator - one demo and one full.
Here on Itch you can download both the demo (free) and the full version. You download the full version on your personal download page. You got a link to your download page in the email when you first purchased the sim. In case you have lost that email you can request a new one here: https://itch.io/support (Under "recover purchases"). Hope that helps!