Calendar.app can no longer add a calendar.

To trouble shoot further you can:

 —A SafeBoot Use safe mode on your Mac - Apple Support will sort many anomalies

Does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, third party system modifications and system accelerations are disabled temporarily.

Login and test. Reboot  as normal and test.  Caches get rebuilt automatically.

This test will tell you if third party interference; most extensions etc are not loaded in safe boot mode. 

—Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support

 This will tell you if it a universal issue or isolated to your user/admin account. 

unplug all non-essential  peripherals when testing

Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

all known to cause issues on the macOS

Gil Dawson wrote:

My Calendar.app Version 14.0 running under macOS Sonoma 14.5 can no longer add a calendar.

Pulling down from File illuminates the "New Calendar >" menu item, but does nothing.

https://discussions.apple.com/content/attachment/eb626fb1-e092-498b-83ea-9d0d0486f63e

It does not expand, as the ">" mark suggests that it should.

Clicking on "New Calendar" does nothing.

Neither do double-clicking, Shift-clicking, control-clicking, Option-clicking nor Command-clicking.

In addition to this anomaly (and possibly related to it) several other calendars, which I created long ago, have disappeared. I do not know how long ago they disappeared; I just today noticed their lack.

I'd appreciate an explanation or suggestions about how to further research this anomaly.

--Gil

Have you quit the App and relaunched it?

and within the app how about the short cut— for a new Calendar(?)

Option Command N