OK, after spending about 30 mins trying to work out what I'd entered incorrectly, and having found this page and added a comment, the solution for me was... (TL;DR, start over)
Go back to the Apps page, start again to create a new app, go straight to the Redirect Url section and add something. This time the "Create app" button became enabled and I then filled out the other fields.
The button is disabled because you have not set all the required values for your app. You will need to set the 'redirect url' before you can create the app.
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OK, after spending about 30 mins trying to work out what I'd entered incorrectly, and having found this page and added a comment, the solution for me was... (TL;DR, start over)
Go back to the Apps page, start again to create a new app, go straight to the Redirect Url section and add something. This time the "Create app" button became enabled and I then filled out the other fields.