If you tried to upload a layer and received an error regarding CRS (Coordinate Reference System), then you are most likely uploading data with projection information that isn't recognized by GeoServer. We suggest that data be referenced to Geographic WGS84 (World Geodetic System 84, the most recent revision), which is a commonly used reference system and works well with Map Mavin. Another possible issue is there could be no projection/datum associated with the data, or a custom projection/datum that Geoserver does not recognize. 

To re-project the data, use a tool like ArcMap's Project to change the CRS to Geographic WGS84 (or use Define Projection to apply a projection if it is missing one).