Let us not concern ourselves here with why some panentheists deny the efficacy of petitionary prayer. Let us just grant for the moment their contention that such prayers cannot be answered.
I often teach panentheism in my intro to philosophy courses, and bring up this denial of petitionary prayer. Inevitably concerns are expressed about this fact. It seems that our modern culture has reduced prayer to the petitionary variety - prayer = asking God for things.
But there are other kinds of prayer that a panentheist would not only accept, but celebrate. There are prayers of adoration, in which the glory of the divine is celebrated, prayers of thanksgiving, in which we express gratitude for the world and our blessings, and contemplative prayer in which we recognize our unity with the divine.
In short, one can have a very rich prayer life without petitionary prayer. In fact, petitionary prayer should probably be thought of as prayer at its most shallow.