Can you help me, why I have such error

with(PDEtools); m := 1/64
r := (8/15)*m
l1` := (40*(1+tanh(n(t))))*(diff(k(t), t)) = -20*r*(1+tanh(n(t)))^2+z(t)/cosh(n(t))^2-(2*(1+tanh(n(t))))*k(t)^2/cosh(n(t))^2 
l2 := 20*(diff(z(t), t)) = -20*r*(1+tanh(n(t)))^2*k(t)+(2/3)*k(t)*(1+tanh(n(t)))/cosh(n(t))^2
l3 := 20*(diff(n(t), t)) = k(t)*(1+tanh(n(t)))
IBC := {k(0) = 0, n(0) = 0, z(0) = 1/3}

Are There  options to solve this problem?

I solved a PDE system and got module() export plot,plot3d,animate,value,settings; ... end module (Got ui(x,t) and ur(x,t))
And after it I try to solve the task
Xs := [0, .175, .789, 1.818, 3.287, 5.183];
n := 1;
interface(prettyprint = 1);
for t1 from 0 by 50 to 50 do
k1 := sol:-value(t1, output = listprocedure);

Help me please, How to get a spatial derivative?After solving a system of differential equations numerically, I got module.Help please, How can I find a  value of the first derivative? 
For example , after solving the PDE system, module() export plot,plot3d,animate,value,settings; ... end module
 So, I need to find diff(U(x,t),x) (for example x=0 and t=0) 

After solving a system of differential equations I got module. So I have a solution for the functions U1 (x, t) and U2 (x, t). Please tell me, how do I now get x * which corresponds to U1 (x *, 10) = max?Help please.

Can you help me, why I get such error,

> restart; with(PDEtools); a := 1; m := 1/64; q := 1+tanh((1/20)*x);

l1 := 2*(diff(ur(x, t), t))+q*(diff(ui(x, t), `$`(x, 2)))+2*a*(ui(x, t)*ur(x, t)*ur(x, t)+(ui(x, t)*ui(x, t))*ui(x, t))+m*ui(x, t)*(diff(ur(x, t)^2, x))+m*ui(x, t)*(diff(ui(x, t)*ui(x, t), x)) = 0;print(`output redirected...`);

 l2 := -2*(diff(ui(x, t), t))+q*(diff(ur(x, t), `$`(x, 2)))+2*a*(ur(x, t)^3+ur(x, t)*ui(x, t)^2)+m*ur(x, t)*(diff(ur(x, t...