You cannot place a Spot Slope annotation on a ramp.
This is unfortunate as displaying a ramps slope is a common thing to do on plans.
But there is a work around, create a sloped floor, add the slot annotation to the sloped floor, then drag it onto your ramp - and it works and correctly displays the ramps slope.
This is a strange work around, and reflects badly on Revit - Spot Slope should simply work with ramps.