I was having fun modifying a code given to me as an answer to a previous problem here, courtesy of user Alex Trounev (Thank you again), when I encountered a certain error which I had never seen before.

Here is the aforesaid code :

```
(*parameters*)
r0 = 0.5;
h = 1;
(Alpha) = 0.8;
(*region definition*)
reg = Cuboid({.5, 0., 0.}, {1., 2 Pi, 1.});
reg3D = ImplicitRegion(
r0^2 <= x^2 + y^2 <= 1 && 0 <= z <= 1, {x, y, z});
(*equation + conditions*)
eq1 = D(u(t, r, (Theta), z),
t) - (D(u(t, r, (Theta), z), r, r) +
1/r*D(u(t, r, (Theta), z), r) -
1/((Alpha)^2 r^2) D(u(t, r, (Theta), z), (Theta), (Theta)) +
D(u(t, r, (Theta), z), z, z));
ic = u(0, r, (Theta), z) == 1;
bc = DirichletCondition(u(t, r, (Theta), z) == Exp(-5 t), r == r0);
nV = NeumannValue(1, r == 1);
pbc = PeriodicBoundaryCondition(u(t, r, (Theta), z), (Theta) == 0,
TranslationTransform({0, 2 (Pi)*(Alpha), 0}));
(*solution computation*)
sol = NDSolveValue({eq1 == nV, ic, bc, pbc},
u, {t, 0, 2}, {r, (Theta), z} (Element) reg);
(*frames=Table(DensityPlot3D(sol(t,Sqrt(x^2+y^2),ArcTan(x,y),z),{x,y,
z}(Element)reg3D,ColorFunction(Rule)"Rainbow",OpacityFunction(Rule)
None,Boxed(Rule)False,Axes(Rule)False,PlotRange(Rule){0,1.5},
PlotPoints(Rule)50,PlotLabel(Rule)Row({"t =
",t}),ColorFunctionScaling(Rule)False),{t,.05,1,.05})
ListAnimate(frames)*)
```

When I run the code, after some time, I get greeted with the following error :

`NDSolveValue::nlnum: The function value {$Failed} is not a list of numbers with dimensions {39639} at {t,u(t,r,(Theta),z),(u^(1,0,0,0))(t,r,(Theta),z)} = {0.0138161,{<<1>>},{-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,<<15>>,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,-4.66626,<<39589>>}}.`

When I click on the three dots next to the error, I don’t find any information on the error like it’s usually the case. I then decide to google some answers.

I found some answers here while also trying to comprehend the error by looking at this and finally that answer here.

So if I did understand it correctly, such error arises when you use NDSolve (or NDSolveValue) to get a symbolical solution to your equation, but problems come up when you try to numerically evaluate it for plotting purpose, or when trying to get a symbolical result with a function that requires numerical values ?

In any case, I do not really understand why I get such error as my plot part is currently between (* … *) so it shouldn’t matter. As for the rest of the code, I do not really see an error but I am just a beginner so…

Anyway, can a kind fellow enlighten me please ?