I checked on this and it looks like the page that should be set up to handle the registration process is not set up correctly. What you’ll need to do is set up a page with the [ESPRESSO_EVENTS] shortcode and set it to be the Main registration page in Event Espresso>General settings under Page settings.
There is more information about how to set up the basic Event Espresso pages in the documentation:
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Support forum for Event Espresso 3 and Event Espresso 4.