2003 Chevy Trailblazer EXT it starts but wont move

What is your question?

Drove to the store one day. Came out, started my truck, shifted into reverse and my truck would not move. Shifted it into drive, it would not move. Mechanic says there are no codes showing for my transmission, and that my problem is electrical. The thing that confuses me is that, he also said the mechanical problem is inside my transmission. I dont understand that.

What makes this problem better or worse?

nothing

How long has your 2003 Chevrolet Trailblazer EXT had this problem?

Just happened one random day