Question: what is the reason unable to draw figure explain pls and fix the problem

> restart;
> u := a[0]-12*sqrt(mu)*(1+lambda*sqrt(mu)*tan(A-sqrt(mu)*(x+y+4*mu*t)))/((a+b)*tan(A-sqrt(mu)*(x+y+4*mu*t)));
              (1/2) /             (1/2)    /      (1/2)                 \\
         12 mu      \1 + lambda mu      tan\A - mu      (x + y + 4 mu t)//
  a[0] - -----------------------------------------------------------------
                                /      (1/2)                 \            
                     (a + b) tan\A - mu      (x + y + 4 mu t)/            
> a[0] := 2;
                                      2
> mu := 1.5;
                                     1.5
> lambda := 2;
                                      2
> a := .5;
                                     0.5
> b := .5;
                                     0.5
> A := 1.5;
                                     1.5
> y := 0;
                                      0
> plot3d([abs(u)], x = -3 .. 3, t = -3 .. 3);
Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct