Configuring a TFS2015 build agent fails because agent pool not found. Why?

The answer from @RoeIF worked for me, but I had a bit of trouble finding the page needed to add the account. Here's a screen grab to help:

The user account running the build process script, needs to be part of the Agent Pool Administrator Accounts.

The error message from the script is misleading.

To resolve this error, I had to add the Windows user which is running the TFS build agent to the "Advanced" access level. This can be done under the root TFS Administer Server (top right gear) > Access levels UI. (http://your-tfs-server-here:8080/tfs/_admin/_licenses) My default access level for the server is set to Stakeholder.