I ran into this same issue, I found the answer at the bottom of the page in one of the help files that came with The power tools. (TFPTCommandLineTool.mht)

Errors

TFPT Error: Unable to determine the workspace

When running tfpt using a command that works with Version Control, you may receive one of the errors:

Unable to determine the workspace 
Unable to determine the source control server

Solutions:

• Run tfpt.exe from within a directory that is already mapped to Team Foundation source control.
• Update your local workspace cache using the tf workspaces command. The tf.exe tool is available in the subfolder Common7IDE of your Visual Studio installation folder. If you launch a Visual Studio command prompt, you can then run the following command (which depends on your versions of TFS/VisualStudio - you should use the version that matches version of TFPT you are using, e.g. if you have TFPT for VS2015, use TF from a VS2015 command prompt):
  • VS 2008-2013 / TFS 2008:
    • tf workspaces /s:serverURL
  • VS 2010-2013 / TFS 2010 (and probably later versions as well):
    • tf workspaces /collection:collectionURL
  • VS 2008 / TFS 2010 (and probably later versions as well):
    • tf workspaces /s:collectionURL
• If you have recently installed Visual Studio 2012, you might have to connect it to the same TFS server/collection you were using in Visual Studio 2010.

When using tf workspaces /s:serverURL make sure you use the right tf.exe!
I had the same problem and was stuck because I used the tf.exe from:

Program Files (x86)Microsoft Visual Studio 11.0Common7IDE

and not the one from:

Program Files (x86)Microsoft Visual Studio 10.0Common7IDE

For Visual Studio 2017 users

I had the same problem when trying to run the tfpt command line on a fresh machine installation with VS215 and VS2017 installed. The latest version for the tfpt tool at the time of writing is from TFS Power Tools 2015. That means the local workspace mapping has to be loaded in VS2015 first before the tfpt tool can find the workspace.

Since my team is using VS2017, I only did the workspace mapping in this Visual Studio version. When I opened the VS2017 developer command prompt to use this tool, I got the Unable to determine the workspace message.

To solve this I opened VS2015 and connect the Team Explorer to the TFS server. It immediately recognizes the workspace mapping that was made under VS2017. After this the tfpt tool works correctly under VS2015 and VS2017 developer command prompts.

I tried all of this and still i got the same error. The error is generic enough to represent multiple issues, i guess..

re-installing TFPT from

https://visualstudiogallery.msdn.microsoft.com/f017b10c-02b4-4d6d-9845-58a06545627f

fixed the issue for me.

Actually, although I believe that in some cases these workarounds may fix things, they do not work in all cases. And I strongly suspect that ultimately this points to what could be considered a bug somewhere in the PowerTools code. The reason I say this is that the tf workspace command has no problem detecting the workspace on my machine from the VS Command console, but from that very same console with all of the same path info, TFPT cannot detect it on my lap top no matter what I try. I just got this laptop and installed VS 2010, 2012 and 2013, along with the respective Power Tools installs, and pointed to a suite of projects that currently spans several TFS 2010 and TFS 2012 instances. Because of this many-to-many relationship, I suspect there is an incorrect assembly reference somewhere, some combination of factors, GAC, Path, Environment Variables, VS Installations, TFS repositories. In each VS version I attempted to run the TFPT 2010 executable from the VS 2010 Command, and so on with the remaining versions, and tried the above workspace cache updates in all their forms... nothing. But using the same project I connected from an old server with VS 2010 and TFPT 2010 installed and ran the same command perfectly. So I think it has to do with what is running on your system, and in the future I will be much more skeptical about running the different versions side-by-side.