Based on your post, I understand that you want to sync SharePoint calendar with Outlook web app calendar.
As far as I know, currently we can sync SharePoint calendar with Outlook desktop app, new events added in the SharePoint calendar will appear in the Outlook desktop app, but there is no way to sync SharePoint calendar with Outlook web app.
Here is my test result, it is the same as yours, there are default calendars and group calendars, but not SharePoint calendar in the Outlook web app.
Meanwhile, to get SharePoint calendar in the Outlook web app, I recommend you give suggestion in the feedback portal and help to make improvements. Many features of current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding and support.
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