Load the finite element package:

In this example a Péclet number has been computed that indicates that the differential equation may exhibit instabilities. The Péclet number is only computed if an explicit mesh is given.

The solution is to use a finer mesh:

In this case, the time-dependent PDE is treated as a three-dimensional spatial PDE because an initial condition is missing. The three-dimensional PDE is convection dominated:

If the intend is to compute a time dependent differential equation as a pure spatial solution then the issue indicated by the message can be avoided by making use of a finer mesh:

If the intend is to compute the time dependent differential equation as a time dependent equation with a spatial discretization then an initial condition given and the time domain should be separated from the spatial domain: