"They are repairing the robot."
qoq lutI'lI' expresses the idea that not only is the robot under repair, but that the repair action is ongoing and has a known stopping point: a fixed robot.
qoq lutI' expresses only that the robot is under repair, but doesn't say anything about the repairs being ongoing. It could also be used to describe a more general state of affairs: for instance, Hoch jaj yaHchaj lujaH. qoq lutI'. ghIq megh luSop. Every day they go to their duty station. They fix a robot. Then they eat lunch. This might be describing a robot repair shop.
Note that English They are repairing the robot might be describing the situation described by qoq lutI'lI' (there's a robot and you're describing the ongoing process of repairing it), but it could also be describing the situation described by qoq lutI' (there's a robot and it's their job to repair it). Unfortunately, this course artificially requires all -ing English verbs to be translated by -taH or -lI' in Klingon. This is not correct Klingon, but the course creators have made it a rule to follow in this course.