Hello jazmyne246,
The person sharing this event may have shared an event that you cannot edit. See the following for more details on the issue and a possible solution: Share iCloud calendars on iPhone - Apple Support
Change a person’s access to a shared calendar
After you invite a person to share your calendar, you can turn on or off their ability to edit the calendar, or stop sharing the calendar with that person.
1. Tap Calendars, tap
next to the shared calendar, then tap the person’s name.
2. Do any of the following:
* Turn on or off Allow Editing.
* Tap Stop Sharing.
Share a read-only calendar with anyone
1. Tap Calendars, then tap
next to the iCloud calendar you want to share.
2. Turn on Public Calendar, then tap Share Link to copy or send the URL for your calendar.
3. Choose a method for sending the URL—Messages, Mail, and so on.
Anyone you send the URL to can use it to subscribe to the calendar using a compatible app, such as Calendar for macOS.
Which means, they may have shared an event that is read-only. You can only read the event, but not edit it. In which case, you'll need to contact the sender and share this information, so you can edit it.
Let us know if that helps out or not.
Take care.