It's an issue with Safari. The same issue happened to me using Safari on a Mac. I had o use another browser (Firefox) to be able to create the app-specific password without the error message you showed. I suspect that using a PC Windows would work as well.
In any case, I did a test on an iPhone with Safari and Google Chrome (another web browser) and both failed. I imagine that using another browser on your iPad will be equally unsuccessful.
Try to find a Mac or a PC Windows to create your app-specific password on appleid.apple.com.