Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello Aleksandar_S
Hi, I'm Karl and will be happy to help you today.
Can I ask where you purchased / got your copy of Office Pro Plus from?
Reason for the question is that Office 2021 Pro Plus is not a legitimate consumer product, it is only available as part of a corporate Volume License agreement.
If you purchased it online from a 3rd party reseller (it is not available from the Microsoft Store), then the key that was provided would not be a legitimate product key and the most likely cause of the problem is that the vendor provided the same key to multiple purchasers which would trigger the message you are receiving and Microsoft would disable the product key.
If the above describes your situation then unfortunately the only recourse you have is to contact the vendor - there is nothing that Microsoft can/will do for purchases from 3rd party resellers.
In addition, all current versions of Office would be associated with the Microsoft Account used to redeem the product key. Does this account still show Office 2019 under <Services/Subscriptions> for that account?
https://account.microsoft.com/services/
Just so there is no misunderstanding, this is a public user-to-user support forum. We're users just like you helping other users. We do not work for Microsoft and have no access to any user account so there is nothing we can do or check from within the forum. Microsoft Support personnel do not staff this forum.