The most common error that people experience is during the creation of the contract first service unit. Below is an excerpt of the error that is typically seen:
Look through the error you received to see if it contains the snippet above.
This error occurs because the package path to your class has not yet been specified. It is OK to proceed with the rest of the tutorial as the rest of the steps will work past this issue. (In short, you must create an implementation class and name it in the src/main/resources/xbean.xml of this service unit.)