You will not be able to install a NPM package on a Webflow hosted site. NPM is a tool that allows you to import dependencies into projects for which you control the build. Webflow does not allow any server side access so you will not be able to do this.
If you want to use external libraries with Webflow you have 2 options:
Pull the library in via a https script include from a CDN (or your own hosting).
Find the build output code for the library and add it directly into your Webflow site via the custom code options. In Github repos, you will often find this in a directory called dist.