Question on Predicates and Quantifiers
The meaning of
$$ \exists u A(u) \land \exists n S(n,\text{available}) $$
is "there exists an active user, and there exists a network link which is available". The conditioning is missing here.

If $P = \exists u A(u)$ and $Q = \exists n S(n,\text{available})$, then we are trying to model "if $P$ then $Q$", whose formal form is $P \rightarrow Q$, whereas your answer is $P \land Q$.

In particular, if there doesn't exist an active user, then $P \land Q$ is always false, whereas $P \to Q$ is always true.