Whenever I try to solve this problem I get an error that I don’t know what it means. The problem is
\begin{equation}
\begin{cases}
\frac{\partial u}{\partial t} = \nabla \cdot (D \nabla u) + \nabla \cdot (\xi u \nabla p)+ (\alpha_A - \beta_{AB})u - \alpha_Au^{2} \ \
u = 0 \ [0, l] \times {h} \ \
u = 1 \ [0, l] \times {0} \ \
\frac{\partial u}{\partial x} = 0 \ \ para \ \ x =0 \ \ e \ \ x = l \ \
u(x,y,0) = 1 - \frac{y}{h} \ \
\end{cases}
\end{equation}