Can I complete the assigned task in the Account Coding workflow notification if the sum total of the distribution amount is less than the line amount?

Yes, if the sum total of the distribution amount is less than the sum of the invoice, provided you're not the last user in the workflow.

If you're the last user, then you're expected to provide the missing distribution amount to complete the task. Alternatively, you can assign this task to another user by using the Reassign action or you can return the invoice back to the initiator by using the Return action.