Buy 1 TB additional storage to my OneDrive for Mac

Have you tried this page?

https://www.microsoft.com/en-us/microsoft-365/onedrive/additional-file-storage

Thank you very much!

Marianne, you are not talking to support here. This is community forum, so no one here can check your account for you. We can only give you advice regarding what you can do, and direct you to the right place if needed.

You mentioned this was for Mac. Is your Msft 365 family purchased from Apple, or directly from Microsoft?

Also, are you the family plan holder?

To add that additional 1TB, make sure you are the family plan holder. If you are just a member in the plan, you won't be able to get this additional 1TB.

Also, if it was purchased from Apple, you won't be able to use Microsoft's website to add it either. You need to contact Apple support, find out if they offer it and how to purchase it.

If the Family plan was purchased from Microsoft directly and you are the plan holder, this can be a temporary issue. You can also reach out to billing support using this method: go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent who has access to your account to help review the issue.