How to set up Apple Watch with missing iPad key from iCloud?

I have:

Mac Air M2

iPhone 16 PRO 1TB (new device)

Apple Watch 11 (new device)

I logged to iPhone by my account and tried to setup the Watch.

During the installation of the Watch, I am ask to write iPad key.

Some time ago (few years ago) I borrowed iPad from school. I do not have it and do not see it anywhere.

However, there is somehow linked this iPad with iCloud (not visible on iCloud devices) and I cannot remove this device neither setup Apple Watch. The iPhone also request the key from iPad for synchronization, but I clearly do not have that iPad and do not have any option to remove it from my devices as it is not there.

How to solve this issue?

[Re-Titled by Moderator]

Original Title: Apple Watch cannot be setup because of request a key for device I do not have and which is not listed anywhere