in your post the "restart" command is written as text

c(x):= is written again as text and "sum(sin(10^k*x)/10^k, k = 1 .. 10^2)" is written as nonexutable math.

rewrite all of this again as math and it will work

You can try this syntax according to help page (Grid,Run)

Grid:-Run(dsolve,[P]);

this gives me results

The source in your equation can not be 0 at infinity, therefore your BC u(infinity,t)=0 is not consistent with this source