"How many times have you used the knife?"
Translation:'arlogh taj Dalo'pu'?
There is no indication in the English that the uses of the knife were accomplishing anything. -ta' should be used when there was an intentional attempt to accomplish something and it was (or will be) successful. We just don't know if that's true in this case and -pu' can be used whether it's true or not, so is there better choice here.
Now, that being said, Duolingo does not provide any context at all and so students may imagine their own context and it's quite reasonable that a student might imagine a context of accomplished intents. Because of that, I've added -ta' as an accepted translation on this sentence.