I've received this error several times now, and it feels like each time it stems from a different issue, so I'm going to update this post every time I receive this error to track the different possible solutions.
1. Restart Visual Studio
This error often happens when adding a new TDS Project to an existing solution, and then immediately trying to compile. In this case, I just restarted Visual Studio and the new TDS Project compiled without errors.
2. TDS Version Mismatch
We use the HedgehogDevelopment.TDS.x.x.x.x.nupkg NuGet package to allow our VSO to compile TDS projects without having to install anything on the VSO server. With multiple developers working on a project, not all are always sync'd on the same version of TDS. Having a lower version of TDS installed on your local Visual Studio than the NuGet package version being used has caused this error before.