Meta requires you to specify the ad beneficiary (a DSA rule) when the audience includes European Union countries. The destination ad set had no beneficiary. Ads Editor tries to reuse the beneficiary from another ad set in the account, or the Page name. If it can't find one, it asks you to fill it in.

How to fix it:

  1. In Meta Ads Manager, open the destination ad set.
  2. Fill in the "Beneficiary" field (and payer, if requested) in the European Union section.
  3. Reprocess in Ads Editor.

If you don't run ads in the EU, you can also remove European countries from the audience.