When you clone between accounts and change the Page, the source WhatsApp number doesn't belong to the new destination Page/account. The WhatsApp Business App is linked to the original Page/Business and can't be moved to another ad account.

This is a WhatsApp/Meta limitation, not an Ads Editor one. In the destination account, choose a Page whose WhatsApp is already linked to that account. When you change the Page, Ads Editor lets Meta infer the WhatsApp number from the new Page, but the number must actually be connected to that Page in the destination. In the duplicate dialog, the WhatsApp number field lists the numbers that have already run in campaigns in the destination account. Choose one of them.