This is a client side path limitation with Team Foundation Server. You can workaround this issue by trying below options.
- Mapping the build definition to shorter build agent path. Go to Source Settings tab in build definition, here shorten the path configured for Build Agent folder.
- Make sure that Build Agent working directory also using the short path. You can check this from Build Menu --> Manage Build Controllers --> Select the relevant build agents and click properties. Here the path configured for working directory should not be too long.