There are many restictions in converting Matlab to C via coder. Coder emits C code on the fly as it parses the .m file. It looks like it assumes that the struct has been fully defined once any of its fields have been used. It would then emit the corresponding C code that defines the C-struct. Since, in C, the whole structure has to be defined in one place, no further changes to the struct are possible.
You will need to get the results beforehand in temporary variables and then put them in the struct.
function [Prm] = Testangles(input)
numangles = input;
angles = linspace(0, 360, numangles);
Prm.NumAngles = numangles;
Prm.Angles = angles;
