There may something wrong with your key. You may need car key programming tool, have a look at http://www.obd2life.com/car-key-programming-c-87.html , hope you can find one that suit your problem.
RepairPal Certified shops can diagnose and repair your 2001 Mercedes-Benz E320. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.