Ok, it gets worse. It is definitely my machine only.
Two colleagues can create custom pages...
I have more licenses than both, including Power Apps Premium now.

It is a user / machine specific issue.
I have tried with both Chrome and Edge. (as have my colleagues)
It works on their machines, but not mine.
I am a system administrator, system customizer.
I created the model app.
I created the dataverse tables for the model app.
I have built PCF controls in this app.
I cannot create a Custom Page.