Question

Flux can not determine directly in non uniform case why

Answer 1

The total of the electric flux out of a closed surface is equal to the charge enclosed divided by the permittivity. The electric flux through an area is defined as the electric field multiplied by the area of the surface projected in a plane perpendicular to the field.

Answer 2

hey

Imagine that your cube is just to the right of a point charge. The field lines from that charge are not parallel, they diverge. So some of them exit the cube through the top, bottom, front, and back faces. The total flux entering the left face is equal to the sum of the fluxes leaving through the other 5 faces.

Or perhaps you are thinking of field lines that are parallel, but decreasing in magnitude from left to right, say E(r)=x^∗(some decreasing function of x)