Hi,
The screenshot provided is a bit blurry, but I can see that the "Use marketing form for registration" option is turned on.
However, it seems that there is no Form hosting script in your screenshot.

It is recommended to check if the script is not there or if it was not loaded during the screenshot.
And also, where do you host this event, a portal or an external web site?
Here are some suggestions:
- Please try to stop the event and go it live again after switching to use an Event Registration Form.
- If you host the event in the Event Management Portal, try to restart the portal from Power Apps.
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya