Assuming you’re certain that you did indeed install using the kickstart script (and not your system package manager or by manually building Netdata), you should be able to get it working by adding
--allow-duplicate-installs to the list of options for the script. You might also need
--reinstall, but I’m not sure about that one.
This check exists to keep people from accidentally installing Netdata on top of an installation done through the package manager, which will typically result in a state that cannot be cleanly uninstalled and is liable to break unexpectedly when the version of netdata installed from the package manager gets updated. Normally it shouldn’t trigger in cases like this, but sometimes something goes wrong during an update and causes this to happen.