The full text of that error code is probably this:
CNET:8015 , The media usage field was set to channels A and B, but a single channel device exists in the configuration. The media usage field was changed to channel A only.
It occurs when you are attempting to schedule a Redundant Media network but there are some single-media devices in the configuration.
One document in the RA Knowledgebase (ID #65433) suggests that the 1784-U2CN can cause this error if an old EDS file is present. Update the EDS (newer than March 2010) and the network will schedule properly.
If the U2CN is not the issue, please describe in detail the devices on your network.