Paprika only supports directly exporting to calendars that have been added to the iOS Calendar app.
However, you can indirectly export to a Google calendar by first adding that calendar to the iOS Calendar app. Then, go to Paprika's Settings -> Meal Planner -> Calendar Export Options and set the Calendar Name setting to same name that your Google calendar has inside the iOS Calendar app. (By default, this is the email address of the corresponding Google account, but you might have changed it to something else.)
You may also need to change the Time Zone option to UTC (instead of Floating) to get the times to line up correctly.