If that's all the person says, no, it's more than likely not acceptable. If there's an extra piece about what a user would do or think in that situation, even if they have no experience, it may be helpful.
I don't know toto! I just don't know. I may know, but I'm just thinking out aloud when I say I don't know, but a good chance I say i don't know because I literally don't know, Or I just use the sentence I don't know because it doesn't make an awkward silence so awkward.. Mmm I don't know 😋
If you don't know, don't say anything. If you try to figure out something and you aren't sure, you can write along the lines "I don't know". But if your entinere answer is "I don't know", then don't even bother.
Of course. Sometimes it's the best answer if it's accompanied by making points so the asker can decide for themselves. People answering questions can never know the situation as well as the asker. But they can bring up points and POVs that the asker may not have thought of.