The fact that Toad is reporting “Error logging into Microsoft Team Foundation Server” indicates that your TFS settings are not set correctly in Toad. Toad 12.6 is an older version of Toad, and thus Team Coding. Much of Team Coding has been rewritten with a new interface for the more recent versions. However, there are a few resources available online to help you set up TFS with Toad. Some of these links refer to even older versions of Toad; however, the interface you’re using should be similar to the ones in the documentation:
Configuring Toad® Team Coding to use Microsoft Team Foundation Server 2010:
How to configure Toad for Oracle with Microsoft Team Foundation Server (TFS) (97366)
Version Control and Team Coding in Toad for Oracle 11.6
and another blog I found on Google:
Toad For Oracle 12.6 and Team Foundation Server 2013: Associating Work Items with Changesets While Performing Check-ins from Toad
Any of these documents should help you in setting up TFS with Toad 12.6. Feel free to let us know if there’s anything else we can do.