By the error message I would guess you’re on an older version of node.js and maybe an old version of npm. And node-red-contrib-modbus wants newer versions.
I am on these versions:
Node-RED version: v0.17.5
Node.js version: v6.11.4
npm version: v3.10.10
If you’re running an emonPi or a Raspberry Pi there is upgrade info here:
https://nodered.org/docs/hardware/raspberrypi
You may want to post a question on the Node-RED forum.
EDIT: Make sure you make a good backup of your system before running any big update (such as this one).
EDIT2: Take a look at this thread: