Question: why taking double derivative with two different independent variable not run?

I did a lot  of time but this time i don't know why not run any one have idea?

restart

with(PDEtools)

with(LinearAlgebra)

with(Physics)

with(SolveTools)

undeclare(prime)

`There is no more prime differentiation variable; all derivatives will be displayed as indexed functions`

(1)

declare(u(x, t)); declare(U(xi)); declare(G(xi))

u(x, t)*`will now be displayed as`*u

 

U(xi)*`will now be displayed as`*U

 

G(xi)*`will now be displayed as`*G

(2)

T := xi = -V*t+x; T1 := u(x, t) = U(-V*t+x)*exp(I*(-k*x+t*w+theta))

xi = -V*t+x

 

u(x, t) = U(-V*t+x)*exp(I*(-k*x+t*w+theta))

(3)

P3 := diff(u(x, t), x, t)

``

(4)

P33 := diff(u(x, t), x)

diff(u(x, t), x)

(5)

P333 := diff(P33, t)

NULL

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