After @Eljay's suggestion of using std::array instead of a C-style array, I changed it and got a new error of Globals::MAX_COL not being a constant variable.

Hence, after changing back to the C-style array, I changed the variable MAX_COL in the namespace to a constant and now it compiles properly.

New Code: namespaces.h

#ifndef namespaces_h
#define namespaces_h

namespace Globals {
    const int MAX_COL = 10;
};

#endif