How to get rid of pop up

Hi OperatorError32,

Thanks for coming to Apple Support Communities for help today with this pop up message "Your response to the invitation cannot be sent".

Have you tried sending this invitation on another network or while the VPN is switched off?

You can also toggle calendars off and back on in Settings > Calendar > Accounts. Doing this in combination with restarting your iPhone may resolve the issue. Restart your iPhone

Let me know how it goes.

Have a fantastic day!

In addition to MoonJ.s suggestion, make sure you have a valid default email account in Settings/Mail (scroll to the bottom).