As you have defined G, it will be a singular matrix - that is, its determinant will be zero, because its rows are clearly not linearly independent. That will lead to a lot of trouble. So your problem is fundamental.
The error message is undoubtedly due to the fact that each F entry is a 1-by-6 array which makes G be 3-by-18 and therefore not square. That I think is easily corrected, but that is not the main problem. Singularity is what you need to worry about.
