I understand from your post that you are receiving an error when trying to create calendar events in different calendars. In a situation like this I recommend checking to make sure the correct calendar is listed as your default calendar under Settings > Calendar > Default Calendar.
If so, toggle the calendar feature for each account under Settings > Accounts & Passwords > Accounts. Tap each account individually and turn the Calendars switch off and then on again.
This article includes information for assistance with the iCloud calendar and may be useful as well:
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Can’t add event to calendar in ios 11.2.5 (15D60)
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