This can happen if you are trying to remove a protected Agent without providing the protection password. Please use the /PASSWORD= parameter and provide the protection password like this:
teramind-remover.exe /PASSWORD=your_pass
If you have forgotten the password, you can change it from the Settings screen:
Click the Gear icon near the top-right corner of the Dashboard.
Select Settings.
Click the Security tab.
You will find the PROTECTED AGENT UNINSTALL PASSWORD field under the Agent removal protection section.