Mupad: ode::solve showing error" equation is not ordinary"

[VED NAMRUB]

I am trying to solve the partial differential equation using MuPad while solving the equation in the third image the last line the differential equation is of the function of z and while solving it for U(z) its shows error as shown in the third image.
When I changed the code the error remains.
The first two images show the complete code

 

[Denis]

I solved it in my head...but then forgot my solution...sorry