Earlier quoted context omitted.
I feel as you should be able to provide an estimate even if it something that you have not completely done before. One should spend some time to gauge how much of this new thing is really "new" and what parts should be easy to figure out. Then, try to look at resources about those unknown parts, and that should allow to provide a rough estimate. And when road blocks come up just communicate early, and then if PM/boss…
Great! Please let us know when cancer will be cured. In other words, "I don't know" is sometimes a complete answer. I don't know. Period. This is supposedly one of the main points of this religion (agile). Some things are unknowable. So you move ahead a bit, and reassess. If you are scrumming, that is usually 2-3 weeks, and don't get me started on that arbitrary limit. But, you try to move forward for a bit, learn so…
Granted that’s more broad than a PM (or humanity) would want, but it is SOMETHING.