Wrote about this before in 2013. Ramps and the Slope Annotation don’t like each other. In the past we could work around their unfriendliness by dragging a working slope annotation from an adjacent floor to the ramp. That no longer works since as far back as 2019.
I’m reasonably sure Ramps have a slope… odd that the annotation doesn’t think so? I wrote another post in 2013 too that describes using an adaptive point family to add some graphics to a floor posing as a ramp which might also permit using the slope annotation on it instead. Yet another thing to experiment with.
Then again it might just be better to ignore the ramp feature entirely in favor of Floors?