The error is for the line
unsigned long myVector[N_ELEMENTS]
as N_ELEMENTS must be initialised during the constructor, the size cannot be known at compile time, late binding impedes this approach.
What you need is early binding.
template< const int i_Elements > class myClass{
private:
unsigned long myVector[ i_Elements ];
};
Instantiate an object like
myClass< 4 > m_Class1;
//rather than
myClass m_Class1( 4 );