I am implementing the TFS Security commands to change a git master branch acess control permissions to DENY for contribute field.
Below is the command I am using:
<According to the error info, you are implementing with a not exist namespace.
You can use tfssecurity /a
command to view a list of namespaces at the server level
The namespace that contains the group from which you want to remove the user or group. You can also use the tfssecurity /a command to view a list of namespaces at the server level, the collection level, and the project level.
More detail info of this command, please refer the link from MSDN: Change groups and permissions with TFSSecurity
You missed a blank space in "Git Repositories" name space. Try following:
tfssecurity.exe /a+ "Git Repositories"