Hello jdajani,
Thanks for using Apple Support Communities. It's my understanding that you're unable to delete an event from Calendar on your iPhone. I'm happy to help.
Make sure your iPhone has the latest software version.
Update your iPhone, iPad, or iPod touch
If you haven't already, force Calendar to close and try deleting the event again.
How to force an app to close on your iPhone, iPad, or iPod touch
If the issue persists, restart your iPhone. Restarting can resolve many unexpected behaviors.
Restart your iPhone, iPad, or iPod touch
Let us know the results. Please include any other troubleshooting steps you may have tried.
Have a wonderful day!