Dear Y. Gilat,
Welcome to Microsoft Community.
I understand that you are prompted for a Microsoft 365 subscription when you open an Excel attachment.
I'd be happy to help, if your account is registered with a third-party email address, New Outlook will not recognize it as a Microsoft account, so it will not recognize the license correctly, it is recommended that you add an @outlook.com alias to your account, and then add the alias to New Outlook, and set it as the primary account.
Please go to https://go.microsoft.com/fwlink/p/?linkid=864833 to add an alias.
You can read the link below for more information.
Add or remove an email alias in Outlook.com - Microsoft Support
If you don't want to add an alias, and your Microsoft 365 is a personal or family edition, then you can use the Classic Outlook application, which should already be installed with the Office suite, and you can search for "Outlook" in the Windows search box to see if it is already installed.
It looks like this. It offers more complete features as well as stability.
In this special season, I wish you all the best—may your holiday be merry and bright! Merry Christmas!
Best Regards,
Jason.YA - MSFT | Microsoft Community Support Specialist