Using Classic Outlook on Windows for personal email, calendar, and contact management
If the option to create a new contact group is grayed out in Microsoft Office Professional 2021, it may be due to the fact that you are connected to an Exchange ActiveSync (EAS) account. Contact Groups are not supported when using EAS, which is commonly used for accounts like @outlook.com, @hotmail.com, @live.com, and @msn.com. In such cases, the New Contact Group button will be unavailable.
If you are using an EAS account, consider exporting your contacts and contact groups to an Outlook Data (.pst) file and then importing them into a new .pst file in Outlook 2021. This way, you can manage your contacts and groups without the limitations imposed by EAS.
For more details on importing and exporting contacts, you can refer to the relevant Microsoft documentation.