Question: figure draw problem why unable to evaluate the function to numeric values in the region pls explain

> restart;
> u := c-6*mu*(1+lambda*sqrt(-mu)*coth(A+sqrt(-mu)*(x+y+mu*t)))/((a+b)*sqrt(-mu)*coth(A+sqrt(-mu)*(x+y+mu*t)))+6*mu*sqrt(sigma*(1-coth(A+sqrt(-mu)*(x+y+mu*t))^2))/(sqrt(sigma)*(a+b)*sqrt(-mu)*coth(A+sqrt(-mu)*(x+y+mu*t)));
            /                (1/2)     /         (1/2)               \\
       6 mu \1 + lambda (-mu)      coth\A + (-mu)      (x + y + mu t)//
   c - ----------------------------------------------------------------
                         (1/2)     /         (1/2)               \     
            (a + b) (-mu)      coth\A + (-mu)      (x + y + mu t)/     

                                                                 (1/2)   
               /      /                                       2\\        
               |      |        /         (1/2)               \ ||        
          6 mu \sigma \1 - coth\A + (-mu)      (x + y + mu t)/ //        
      + -----------------------------------------------------------------
             (1/2)              (1/2)     /         (1/2)               \
        sigma      (a + b) (-mu)      coth\A + (-mu)      (x + y + mu t)/
> c := 1;
                                      1
> mu := -1.5;
                                     1.5
> lambda := 1.5;
                                     1.5
> a := 1;
                                      1
> b := 1;
                                      1
> A := .5;
                                     0.5
> y := 0;
                                      0
> plot3d([abs(u)], x = -10 .. 3, t = -10 .. 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