The ERROR LED indicates the ATLAS is receiving errors on the particular module. You can look under the MODULES at the MENU for slot 6 and look under the performance to see what types of errors it is receiving.
If module 6 is a T1 module it may simply be CSS errors indicating clock slips. If that is the case you can see what the ATLAS has configured for its primary timing source and if the equipment off slot 6 should be taking timing from the ATLAS. If it is another type of error you may have to see if it is a back cable, and possibly try a loopback test to verify the ATLAS interface.
Hope this helps,