When the cart maximally compresses the spring at the bottom of the fixed track, the cart's:

At equilibrium position F_net = 0 and velocity is maximum but after that spring force increases and net force becomes in upward direction therefore at maximum compression acceleration will be non-zero.

Let compression in spring is x.
At maximum compression $\frac{\mathrm{dx}}{\mathrm{dt}}=0\Rightarrow \mathrm{v}=0$.