Why do I get the "Device Already Associated With an Apple Account" Popup on my iPad?

You can read what Apple says about it, here:

From: View and remove your devices that are associated with your Apple Account for purchases - Apple Support

"When you try to associate a device that was previously associated, you might notice a message that says, ‘This device is associated with another Apple Account. You cannot associate this device with another Apple Account for [number] days.’ You might need to wait up to 90 days to associate the device with a different Apple Account."

Somehow another account was used to download something (an app or media) on that iPad in the last 90 days. You are welcome to contact Apple about this but it would not surprise me if they just tell you the same thing. :-)

Refer to this page for Apple Support features ➔ Contact - Official Apple Support

Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.

This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support For some countries look under “Other” listing at the end of each region. Not every country has telephone support.