Entering calendar events

1. Restart your iPhone, and or Force restart your iPhone.
2. Try to Quit and reopen Calendar.

What Calendar Accounts are you using in Settings>Calendar>Accounts>?

Edit: FWIW, this is what I see in the "Day View," and the + appears on the top right:

The Support Article states:

"Add an event

1. In day view, tap  near the top left."

Please refer to Create and edit events in Calendar on iPhone - Apple Support.

williamfromhavre de grace wrote:

Right, not the issue.

...but other than saying "Can’t enter calendar events," you have not really described the issue.

You are welcome.