The Approved checkbox displays only if the Require Approvals on Journal Entries Accounting Preference is enabled at Setup > Accounting > Accounting Preferences > General tab > General Ledger section.
The Approved checkbox is not displayed if you have enabled the Journal Entries Approval Routing Accounting Preference. This preference enables you to use SuiteFlow to create a custom journal entry approval routing workflow. If the preference is enabled, an Approval Status Field would be displayed instead of an Approved checkbox. In this case, the Approved checkbox cannot be mapped during CSV import as it is not showing on the form. Mapping the Approval Status field is a current System limitation. This is still a pending Enhancement Request logged under enhancement #434764