Can I edit a current Task so that the option 'require learners to complete again' is selected?

No - This option can only be edited for recurring Tasks.

Tasks that have already been assigned will not be able to have this option changed as it is not architecturally possible.

Completion happens at the time of issuance, so we’d be unwinding completions that have already been recorded.

If you find yourself in this situation then best practice will be to reissue the Task.