2008 Milan, code U0073, where do I begin in order to fix the problem? It also has code P0020

U0073 Control Module Communication Bus Off Digital Signal Processor REFER to Section 418-00 . U0073 Control Module Communication Bus A Off EATC Module REFER to Section 418-00 . U0073 Control Module Communication Bus A Off SJB REFER to Section 418-00 . U0073 Control Module Communication Bus A Off TCM (FNR5 Speed Transaxle) REFER to the Powertrain Control/Emissions Diagnosis (PC/ED) manual. U0073 Control Module Communication Bus A Off TCM (6 Speed Transaxle) REFER to the Powertrain Control/Emissions Diagnosis (PC/ED) manual.
Your best bet , take it to a qualified repair shop . You do not have the knowledge or diagnostic test equipment to diagnose this . Do you know what C.A.N. - controller area network is ?
Communications Network Vehicle communication utilizes both medium and high speed controller area network (CAN) communications. CAN is used for many modules to communicate with each other on a common network. CAN in-vehicle networking is a method for transferring data among distributed electronic modules via a serial data bus. Without serial networking, intermodule communication requires dedicated, point to point wiring resulting in bulky, expensive, complex, and difficult to install wiring harnesses. Applying a serial data network reduces the number of wires combining the signals on a single network. Information is sent to the individual control modules that control each function.
The vehicle has 2 module communication networks:

• Medium speed (MS) CAN
• High speed (HS) CAN

Both networks are connected to the data link connector (DLC). This makes diagnosis and testing of these systems easier by allowing one scan tool to be able to diagnose and control any module on both networks from one connector. The DLC can be found under the instrument panel between the steering column and the audio unit.

016 CAN Communications

P0020 - Intake Camshaft Position Actuator Circuit/Open (Bank 2) Description: See the description for DTC P0010. Possible Causes: See the possible causes for DTC P0010. Diagnostic Aids: See the diagnostic aids for DTC P0010. Application Key On Engine Off Key On Engine Running Continuous Memory All GO to Pinpoint Test HK .