How to prevent ’This device is already associated with an account’ prompt?

A77bruinB wrote:

I logged into my friend’s AppleTV account

This was a huge mistake.

We still don’t know what device(s) is/are involved.

Thoroughly cleanse it/them. Perhaps this applies.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

A77bruinB wrote:

Why was it a huge mistake if I don’t have AppleTV and wanted to watch something?

Because each Apple devices must be used by no more than one Apple Account. The Apple Account using the device may be switched when selling the device by following the directions I linked above.

If I follow your link, it will wipe everything clean …

Yes it will. Backups must occur before catastrophes.

If you have a backup from before the device used a second Apple Account, after following the link provided above:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Without such a backup, I really don’t know if this solution would work.

You have asked us to guess all the steps you took to cause this result.

It seems reasonable to assume you had some device(perhaps an iPad) on which you switched Apple Accounts, likely without thoroughly cleansing the device first.

Perhaps if you tell us each and every step you took, along with the device name, model, operating system version and all other pertinent details, we can assist further.