Actually I already saw the npm is installed after I installed the nodenv, following these guidelines:
https://www.openproject.org/development/setting-up-development-environment/

I checked the environment by typing npm —version and got the result:
3.10.10

However, my problem happened with one of the steps after that:
npm install
which is to install the node_modules. I got the error message:
npm WARN lifecycle openproject@0.1.0~postinstall: cannot run in wd %s %s (wd=%s) openproject@0.1.0 cd frontend && npm install /root/openproject

I almost fixed it, only with a few warnings:

I tried the install with option —unsafe-perm
npm install —unsafe-perm

then it works and ran for about 10 minutes, then pump out some warnings:
npm WARN openproject-frontend@0.1.0 No repository field.
npm WARN openproject-frontend@0.1.0 No license field.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.1.3 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.1.3: wanted {os (current: {os)

I hope these warning don’t matter too much!!