You could include an “I don’t know” as the left-most option in the sliding scale, you’d have to communicate that really clear to the survey responders though so they know it’s there. You could even default it to be on the “I don’t know” answer if you wanted.
Another option is to use *component to put an optional button beneath these sliders that says “I don’t know”.
I do wonder what the purpose of the “I don’t know” is though - would you rather have a person choose that than make their best guess of the probability? In our surveys, we usually prefer the latter.
More generally, the preferred way to ask for probabilities is like the image attached (note it is cutoff, it doesn’t show the full range of options) rather than a slider.
One question per page is preferred except in the case where I expect they will need to adjust a set of questions as a group (e.g. their answer on the second one means they might revise their answer on the first one) but otherwise one question per page tends to produce the fastest survey with least confusion, that’s why we use it as the default.