This sequent is provable.

\begin{logicproof}{1}
  p \land q 	& \prem\\
  \begin{subproof}
  \neg(p \land q) & \assum\\
  p \land q & $\copying 1$\\
  \bot & $\nege 2,3$\\
  r & $\bote 3$
  \end{subproof}
  \neg(p \land q) \imp r & $\impi 2-5$\\
  ((\neg(p \land q))\imp r ) \lor ((q \imp s) \land t) & $\ori{1} 6$
\end{logicproof}