I would like to know where is this error code located in the AOT. Would like to know the path to understand the structure and develop custom code.
Transaction has been selected, for settlement, although settlement type: none was selected
I generally use one of two methods to locate message strings.
Provided the cross reference is updated (it should be in dev) use the "Label editor" to to search for then string, see this answer.
Put a breakpoint in top of info.add
method, disable CIL if needed, then rerun to get the error message invoking the debugger, see this answer.