The Analyze import process validates the presence of any library nodes that are specified in the legacy data flow (.brg) file.
Missing library nodes will generate errors during the import process. You can continue to import the legacy data flow but, when the data flow is opened in Analyze, the missing nodes will be replaced by placeholders in the data flow and connection information may not be present.
You could alternatively edit the base libraries in BRE to remove the 'missing' library and save the modified graph (with a new name) and then attempt tp import the legacy data flow into Analyze. This is the way to go if you have referenced a custom library in the graph but you are not actually using any of the nodes that are included in the library.