Why can't I mark an employee's OKR as complete?

OKR's can only be marked as completed once all the associated key results are progressed 70% or higher. SMART objectives, on the other hand, can be completed at any stage (including 0%).

You may want to consider either extending the due date of the OKR, or delete the key result(s) less than 70% so that you can mark the rest of the OKR as completed.