Hello everyone,
We have the latest progress and temporary solution for this issue here.
Our team reproduced and found out the cause of the problem: When a third-party IMAP/POP email accounts, such as Gmail, Yahoo, is set as the primary account of the new Outlook, this problem of not being able to create calendar events will be triggered. However, only the primary account cannot create events, and other accounts in the application can create events normally. This problem does not occur for primary account of Microsoft domains (outlook.com, hotmail.com etc.).
Solution: Please add another mailbox that is not frequently used as the primary account, so that your current mailbox will be able to create calendar events normally. Click [Settings] - [Accounts] - [Manage] - [Set as primary account].


If it is convenient, OP @Lobengula325 could you please click "Yes" for this reply to highlight the information so that other users who encounter this problem and come to view this post can learn about the latest progress faster. Thanks in advance!
Please relax for the time being and try to see if the temporary solution I provided can solve the urgent problem. The fix for this problem is on the agenda, so please be patient. Finally, everyone is welcome to provide feedback and share insights, and this post will remain open.
Best Wishes,
Stan - MSFT | Microsoft Community Support Specialist