I have recorded a video of creating a new Apps Script file, and then creating a new GCP project to associate with the Apps Script file.
There were at least 2 error messages that were resolved by refreshing the page. And after the new GCP project was created it did not show up in the list until I refreshed the page. In this process, I created the Apps Script file first, but you could create the GCP project first, and set up the configuration page before entering the project number, which would avoid the error messages. However, first time users, aren't going to do that. They don't know to do that. They are going to create an Apps Script file first, and then go from there.