I am getting this message when attempting to upload a mod. I just recently re-installed the DevKit, and previously I had been disabling the SteamGuard, but I wanted to just have it work the right way. Continually disabling and enabling SteamGuard has made my steam act quite weird. I have attempted to access this through the steamcmd prompt, but that has not worked very either and gives the same error (even when i enter the correct steam password with my password)
Just use the email guard code system.