An integrated threat protection solution designed to detect, investigate, and respond to cyber threats across Microsoft 365 services.
Hi,
Regarding Office 365, the issue concerns the version of Office you are using; you should update to version 16.83 to use it.
Try update Office: https://support.microsoft.com/en-us/office/lifecycle/officeinstall/update-office-for-mac-automatically
or
You can download Office via the link above: https://learn.microsoft.com/en-us/officeupdates/update-history-office-for-mac
This might not be a technical fault, but rather due to the disabling of editing and saving features for Office 365/2019 on older Macs, as explained in the link below:
Please consider rating the question, and Access the community at Microsoft Q&A if you have further questions.
Thanks