That appears to be the standard passcode screen when one is setup to lock the ipad when not in use. The other thing is that what you may be envisioning as an obscure ENTER button may just be something that has been blurred in the background. There is no ENTER button when a passcode is entered. You just press enter on the iPad’s keyboard.
The above still does not explain why you received a passcode screen when a passcode was not set. I cannot explain that unless your iPad was restored from a backup file of a device that had the passcode set.
Now that you are at a point where there is a passcode and you don’t know of one, the only option that you have is to reset your passcode. You will need a computer to complete this task. You can reset your passcode by following the instructions in this article by Apple ➔ How to reset your passcode on a disabled iPhone. All your data on your iPhone will be wiped clean in the process.
You may also choose to begin by contacting ➔ Apple Support first.
Axel F.