Hi,
Thank you for the sharing.
But Campaign is not listed in Marketing by default. In Marketing app, the entrance is in the form of Marketing List.

Maybe you have customized the app in a previous environment.
On the other hand, the entity is listed in the Sales app by default.

If you're sure you haven't done a customization before and can't find the portal now, you can also turn to MS ticketing for more help.
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