If your source or target is a numbered list, try creating a line item that is the code of that numbered list and map your list to the code. It's best practice to map Numbered lists with code. That should fix your issue.
As you are using "Match Name or Code" for the list I assume that you are not using Numbered List.
In a standard list, one of the reason for system mapping failure could be the usage of same name in parent hierarchy and child hierarchy. For example, if there are Region and Sub-region hierarchy and both the lists have "Europe" as a member (while creating the list member Anaplan will not give duplication error as its unique for that list), then the system mapping will fail. Same rule applies for code too.
If you can give the screen shot of the mapping or the list then it will be easy to identify the issue in your list.