By default users are locked out of the main system terminal but there are a few way to get in.
First way is to enable port 22222. ssh’ing into this port will give you the system terminal. From there you can navigate to the NR container.
Second way is to use this version of portainer. With that you can go through the gui to the container and open a terminal.
Note: to the OP his is going to give you full access to the system, changes you make from there can result in an unrecoverable instance. Only proceed if you are sure you know what you are doing and make a backup JIC.
E. Also it’s probably best not to remove nodes from the command line, use the palette manger instead.