Please try the following.
You should Force close the calendar process on your Mac computer using a terminal. To do that, open the terminal app and run the below-mentioned command (To open Terminal, Press Command + Space and Run below two command lines in a terminal and hit the return to run).
launchctl stop com.apple.CalendarAgent
launchctl start com.apple.CalendarAgent
Copy the command line, paste it into the terminal window, and hit the return to run command on your macOS.
It worked for me. Please let me know if it worked for you.