Hi Marco,
In a MATLAB Function block, you can only use the subset of the MATLAB language and language features that are supported for C/C++ code generation. However, you can define the function using coder.extrinsic in case the function is not supported for code generation. Since extrinsic functions execute in the workspace during model simulation, your error will be resolved. Try adding the following to your MATLAB Fcn block
coder.extrinsic('makima');
You may also consider preallocating some memory to Fx1 using coder.nullcopy. Try adding this line also, if you face any errors.
Fx1 = coder.nullcopy(zeros(s)); % replace s with the expected size of Fx1
