Translation:I've taken off my belt.
This is a design feature of the course. Verbs with the suffixes -pu' or -ta' have to be translated into English with a verb in the perfective aspect, and vice versa, even though there isn't an exact 1:1 correspondence in usage between the perfective aspect in English and the perfective suffixes in Klingon. It is a similar deal with -taH and -lI', which must be translated with the continuous aspect in English.
English doesn't have verbs in perfective aspects. What it has is perfect tenses:
Present perfect: have verbed
Past perfect: had verbed
Future perfect: will have verbed
English perfect tenses are usually, but not always, perfective in aspect, so translating from English perfect tenses to Klingon perfective aspect is usually perfectly fine (no pun intended).
An English perfect tense means that an action happens BEFORE the time context in question. I have eaten; the eating occurred before now. "Before" is a tense concept: it tells you WHEN something happens. But Klingon verbs have no tense; the verb itself does not tell you when something happens. Klingon verbs have aspect, which tell you HOW something happens, your relationship to the event and the manner of the event's unfolding.
English perfect tenses usually get translated into Klingon perfective aspect because an event that happens before now is probably complete, and Klingon perfective is used to describe an even that is complete from your viewpoint. But English perfect tenses are not the only way English expresses perfective concepts. For instance, I ate breakfast, referring to the specific breakfast I ate this morning, is a perfective concept: from my viewpoint, the act of eating is completed. But it's not in an English perfect tense. Yet being perfective, it is exactly equal to Klingon nIQ vISoppu'.
The "design feature" Michael mentions is that the course will usually not recognize that something like nIQ vISoppu' can be translated as I ate breakfast. It'll require you to put it into English perfect tense: I have eaten breakfast. Worse, it will force you to translate I ate breakfast as nIQ vISop, which is explicitly NOT perfective. The only way I ate breakfast can fail to be a perfective sentence is if you're saying something like, Every day for a year I ate breakfast in bed qaStaHvIS wa' DIS, Hoch jaj QongDaqDaq nIQ vISop.
So unfortunately, partly for simplicity of the software, and partly because a lot of Klingon experts confuse perfective and perfect in English, this course will teach wrong uses of Klingon aspects like -pu'. For now, there's nothing you can do but try to learn the right meanings and plug in the wrong answers to get past them.
This is something I really despise about this course. It's teaching you semantics of Duo rather than the meanings of the language. I would NEVER say "I have taken off the belt" as a preference to "I took off the belt" unless I was in an argument with someone who had claimed I hadn't taken it off and I wanted to put emphasis on "have". There is no difference in meaning in English nor any semantic difference so I will continue to push for proper acceptance and they will have to accept all my "my answer should have been accepted" submissions. I wish it would annoy them as much as it does me.
Okrand gives both examples and I would never assert have is wrong. But not having have is also correct: “It is often translated by the English present perfect (have done some-thing). Daleghpu’ you have seen it (legh see) vIneHpu’ I wanted them (neH want) qaja’pu’ I told you (ja’ tell)”. The distinction between pu’ and ta’ being intent which cannot be distinguished in the English translation. So duolingo is incorrectly flagging these as incorrect translations.