Dear Barry Kingston
Thank you for posting to the Microsoft community.
I understand that you cannot make adjustments or add events to the calendar in Outlook (New).
Recently, many users have reported the same issue. Based on their feedback, they've experienced issues when using non-Microsoft email accounts. Some users mentioned that after adding the account again, they were able to add events for a short period of time, but the issue persisted afterward.
We have collected and submitted solutions for the problems. You can also provide feedback on this feature request directly from the new Outlook (Help --> Feedback --> Report a Problem).
Contact support and provide feedback in new Outlook for Windows - Microsoft Support
You can mark my answer as "Yes" so that more users can learn more about this topic. The more attention users pay to this issue, the more attention it will receive.
Thank you very much for your contribution.
I will share any updates with you if there is any progress.
Best regards
Simon.W - MSFT | Microsoft Community Support Specialist