first of all I think you modeled it badly! Rhino should fillet this object correctly! You probably already messed up the original curve somewhere, before extruding it!
or your scene tolerances are too big! thumb of rule: the tolerances have to be smaller than your fillet radius! lets say you wanna do a fillet of 0.05, but your tolerances are 0.1, it might fail! Use 0.01 instead!
If you prefer to fix your model:
easier but altering the model >try to fillet first the vertical edge (the one where your horizontal fillet fails!) by a small radius. for a simple model likt that it should work!
the way of the pro’s: fillet, accept the failure. you end up with a extended surface of your fillet. extend also the one that ends up where the fillet fails. now the two surfaces of the fillet should intersect! split them, delete the extended parts and try to to split the whole model with all the filleted surfaces!
should work!
otherwise post your model here, we can fix that