I had the same problem, next tried renaming it to npm.cmd which didn't work neither, then tried a full path, which worked at first. But next thing I knew, the npm script had dependencies to other processes which couldn't be found neither. It took me a while to realize what was going on, but it made perfect sense.
In windows, whenever you install a command line application it adds its installation folders to the PATH variable. And that is also the case for node, npm, ng and many more. There is a tricky thing with the PATH variable though, it doesn't just refresh for active terminals or processes. So, whenever you install something to it, you need to close and reopen your terminals.
And actually the same goes for your running jenkins. It also needs to be restarted in order to have its PATH variable updated.
So chances are that you just installed node or npm but you didn't restart your system yet, and didn't restart your jenkins server. In that case, it will give you exactly the error mentioned above.
All my issues were solved simply by restarting the jenkins service.