Unable to mark time entry as extra

A user is not able to mark time entries as Extra.

This happens if your security permissions restrict you from doing so, e.g., users with the Time and Expense Only security profile. If you want to allow marking time entries as Extra, you need to customize the security permissions from Settings > Access & Permissions > Security Permissions wherein you check the option Allow to mark entry as extra under Time Entry. Check CORE Help Center for details.