What does a line number of N/A mean in AMP?

A line number of N/A means that the violation does not have a line number reference readily available. Violations that don't have line numbers available are either manual violations where a description of the violation is provided, pattern violations where the pattern in violation is identified across some set of pages, or global violations where the issue is endemic to the system as a whole. In all those instances it's best to look at the linked best practice and the element description to determine what the specific issue is. In all instances the auditor will have provided specific comments about the problem in the "element" field for the issue, which should give you an idea of where to look for the problem.