Designing 3D parts is hard enough, and while parametric modeling has uses... come on.
I would love to go back to FreeCAD, but for now I'm using Onshape (I run Linux, so Fusion isn't an option).
As I understand it, there are no other open source alternatives around. On the commercial side there are some, perhaps the foremost being the venerable Parasolid, which is used by Onshape, Solid Edge, Solid Works, Siemens NX, Shapr 3d and others.
Creating a solid 3d kernel is hard. Parasolid is from 1986.
I'm guessing you're trying to set a fillet which would completely consume one of the faces adjacent to the edge being filleted. In these cases I've found that a workaround is to make the fillet 0.001mm smaller than the size which would consume the entire face. You end up with a very very small amount of flat area but it's so small it doesn't show up during machining or 3d printing.
The workaround mentioned isn't a magic solution, it fails in a bunch of more complex cases too. Chamfers are a complicated operation and predicting valid values across of the operation in order to set a maximum value is, as far as I'm aware, in a similar region of difficulty. It would also be unfeasible in many cases to attempt the actual operation across a wide range of values. It's not so much a choice as just being hard to do.
These are almost all caused because they use Open Cascade under the hood