Klingon plurals are optional
There's someone going through the Klingon course right now, dutifully pointing out at each sentence that the plural of tlhIngan is tlhInganpu' or the plural of qoq is qoqmey. The person is correct, but the course is trying to teach them that the plural suffixes are almost always optional in Klingon.
Think of that famous exclamation, "tlhIngan maH!" It means "We are Klingons!" and it is not ungrammatical. It's obvious that when the subject is we that the the Klingons are plural, so -pu' is not used. I hope they will come here and learn this, and not become frustrated and think the course is wrong.
There are very few places where you'd be wrong to put a plural marker on a noun. One of them is if you're asking "how many?" Always tlhIngan 'ar Dalegh? (How many Klingons do you see?) or qoq 'ar Dalagh? (How many robots will you disassemble?)
batlh peghoj, HaDwI'!
Some stylistic guidelines. The following are not rules, just some ideas on speaking or writing clearly and effectively.
Klingon often makes the plurality of a noun clear even if you don't use a plural suffixes. cha' taj vI'uch I hold two knives. taj is obviously plural because there are two of them. qama' DIHoH We kill prisoners. qama' is obviously plural because the DI- prefix leaves no other interpretation. In these cases it is usually fine to leave off the suffix. I usually leave off the suffix unless I'm trying to be very careful and clear.
When the possibility of ambiguity might lead to confusion, use a plural suffix. Qel, 'oy' 'uSwIj Doctor, my leg hurts / Doctor, my legs hurt. Your doctor needs to know whether one or both of your legs hurt. Use 'uSDu'wIj for clarity if both your legs hurt. Specify WHICH leg hurts if only one hurts: Qel, 'oy' nIH 'uSwIj Doctor, my right leg hurts.
Sometimes a noun's plurality can be ambiguous and it doesn't matter. tlhInganna' SeymoH QeH Anger excites a true Klingon / Anger excites true Klingons. Both singular and plural express the idea just as well. In these cases I actually recommend not using the plural suffix, as singularity and plurality are not really relevant to the concept.
Again, these are just stylistic guidelines, not grammatical rules. If you put a plural suffix on a noun that is obviously plural anyway, or leave one off in a situation that leads to ambiguity, you haven't done anything wrong.