Thank you for writing.
The error you get is caused by the fact that all objects assigned to the AssociatedObject property of RadPrintDocument must implement the IPrintable interface. This is the interface through which the object and the print document communicate. If you want to pass a RadTextBox to the RadPrintDocument you will have to create a class that inherits from the text box class and implements the IPrintable interface.
I hope this will be useful. Should you have further questions, do not hesitate to write back.
the Telerik team