Hello JadesOne,
We'd love to assist you with using your Calendar app on your iPhone. First, does the issue occur after restarting the iPhone using the steps here: Restart your iPhone ?
Also, is the issue only occurring when adding events to a specific calendar? You can view a list of your calendars by opening the app and choosing the Calendars option at the bottom? If you're unsure, then let's try adding it to a different calendar if you have multiple ones setup.
Lastly, let's make sure the device is using the latest software version to prevent any issues being caused by older software. You can check for updates and process them using the steps here: Update your iPhone, iPad, or iPod touch
Take care