The Unkoles offers a beautiful Pshat.
In truth, why did Hashem say מה תצעק אלי, why would Hashem not want the Tefillos of the Yidden?
He learns Pshat that Hashem was telling Moshe, the Tefillos that you asked for, came before me and I accepted them.
מה תצעק – That what you asked for, אלי – came before me (and was accepted). Now, דבר אל בני ישראל ויסעו talk to the Yidden to move!