Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello Zbyněk P,
Welcome to Microsoft Community.
I am a Microsoft 365 user just like you, I hope you are doing well?
Did you receive this error when redeeming this product key or you are trying to activate Microsoft 365 apps using product key?
If you have Microsoft 365 product key, you need to redeem the product key to your Microsoft account in order to install and activate Office on your PC.
To redeem the Office 365 product key:
Step 1: Go to www.office.com/setup or Microsoft365.com/setup.
Step 2: Sign in with your Microsoft account, or create one if you don't have one.
Step 3: Enter your product key, without hyphens, if prompted.
Step 4: Follow the prompts to finish the redemption process.
Once you have redeemed the product key to your Microsoft account, you can then install and activate Office 365 using the Microsoft account associated with this subscription.
Source: https://support.microsoft.com/office/0a82e5ae-739e-4b92-a6f4-2ec780c185db
Please let me know if you need more assistance.
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Warm Regards