What happens if you go to your .node-red folder and install it manually npm install node-red-contrib-ui-contextmenu
How did you install nodejs and node-red? I believe the error means that when you try to install from inside node-red that it cannot find npm for some reason.
Did you go to your .node-red folder as suggested before installing it?
Look in the startup log you posted to see where that is.
Also, if you want to avoid the install problem happening again then you must answer the other questions about how you installed node-red and the questions from @Steve-Mcl
@Steve-Mcl yes you i use the same account , i add to path node directory and now can install correctly the contrib-ui-contextmenu , now is visible in the Flow... Thanks very much..
Solution : Path of node directory not included in the Path
Thanks @Steve-Mcl for precise information...