Erik Kostson
Ansys Employee
Ok, there are none option for rigid dynamics - that is fine.

It is a bit confusing, you are showing rigid dynamics, and the error message says Solution -> Transient -> Force -> Reaction.
So FSUM is used in APDL solutions, say static or transient structural systems(flexible stress analysis not rigid dynamics), do not think this (FSUM) exists in rigid dynamics systems.

Can you show exactly what you are doing because it is very confusing - please show all systems (rigid dynamics and so on), and perhaps someone can understand better.