After trying to troubleshoot this very same issue I discovered the message from Microsoft below:
Changes to cloud storage
Starting February 1, 2023, cloud storage used across Microsoft 365 apps and services will include Outlook.com attachments data and OneDrive data. All data will continue to be protected with Microsoft’s comprehensive set of security features.
This update may reduce how much cloud storage you have available to use with your OneDrive. If you reach your cloud storage quota, your ability to send and receive emails in Outlook.com will be disrupted.
To ensure we offer the best experience, the cloud storage changes and new quota bar will gradually roll out on or after February 1, 2023, across your app settings, Windows settings, and Microsoft account. If you do not see the new storage experience in your settings or Microsoft account, be sure to check back in the coming weeks.
I have been using Microsoft Office 2013 for years (and enjoyed using the Outook mail software)
I was not even aware that files were being stored in OneDrive cloud storage.
When I clicked on the pink ribbon advising I needed more storage space there was an option to view OneDrive cloud storage. There were files stored there from 2015/2017 I deleted them all. A person is also presented with different options to 'purchase' increased storage capacity.
I will continue to delete what I can, but I am going to start using my (Google - gmail) account from now on.
I spent way too many hours trying to fix my outlook software and was glad when I finally found out what is truly happening.