Translation:I have made a mistake.
I make mistakes in colloquial English doesn't mean I continuously make mistakes; it means that the speaker is not perfect. I am making mistakes could be interpreted as continually, or at least, occasionally, erring, while I always make mistakes or I constantly make mistakes would be interpreted as one's habit, tendency, or potential to make mistakes without reference to any particular mistake.
jIQagh is indeed sufficient for I make mistakes, but it doesn't specify whether you've made one or more errors.
You might make the distinction by being more specific about how you err. rut jIQagh Sometimes I err; pIj jIQagh I often err; reH jIQagh I always err. Or if I make mistakes is meant to indicate your potential to make mistakes, you might use jIQaghlaH I can err.
This is one of those instances where the grammar of English and Klingon don't match, so however you translate it will have a slightly different nuance than you started with.
Same answer: Whether there was one mistake or many is not specified in Qagh. jIQaghpu' can mean I have made a mistake, I have made mistakes, I made a mistake, I made mistakes, I will have made a mistake, I will have made mistakes, I had made a mistake, or I had made mistakes.
Think of the word Qagh as meaning err and you'll get it.
And just to be clear, I disagree that it is the best translation and that Duolingo is teaching wrong Klingon. There is a huge amount of overlap and we are not teaching the edge cases. We can't teach everything at once and have decided to leave those edges for you to learn after completing the course. I believe David disagrees about the specific areas and amounts of overlap. I suggest you consider his advice, but with it with a grain of salt, just as you should mine.