I just spent way too much time trying to login to a VSTS instance from VS. Turns out to have something to do with how Microsoft Account is configured in Visual Studio. So, if you get this error:
“You cannot use this account for this purpose because it belongs to an organization. Please choose a different account or sign up for a new one.”
Open a cmd prompt as administrator and run the following:
reg add HKCU\Software\Microsoft\VisualStudio\12.0\TeamFoundation /v LegacyMSA /t REG_SZ /d True
Restart VS and try to signin again.
You should now see this auth dialog