Hello Scott,
I'm not sure about usecase of your query (used aggregation function doesn't make much sense to me for such report), however this should work for you:
SELECT TOP 100 IFT.InterfaceID, Sum(IFT.InAveragebps) AS SumOfInAverageBPS
FROM
Orion.NPM.InterfaceTraffic IFT
WHERE
IFT.DateTime >= '2014/01/21' AND IFT.NodeID = '712'
GROUP BY IFT.InterfaceID, IFT.DateTime
ORDER BY
IFT.DateTime
The reason is simple. You can't mix aggregate functions with other not-aggregated columns without usage of them in GROUP BY statement. Good examples are e.g. here . The error is returned directly from SQL server, not failing at the SWIS endpoint. More informations about the error you can always find in the Orion.InformationService.log (default location for SWISv3 logs is c:\Users\All Users\Solarwinds\InformationService\v3.0\).
I hope it helped.
Regards,
Honza