This warning means that the package does not have the repository field in the package.json. This field is used just for informational purposes. Suppose you are an author of a package and you are getting this error, you can write the below code in your package.json to remove this warning.
"type" : "git",
"url" : "Your_Package_URL"
According to NPM, repository means the place where your code lives. This is helpful for people who wants to contribute. If the git repository is on GitHub, then the npm docs command will be able to find you.
You can also check the issues already logged https://github.com/npm/npm/issues/3568. Please Like and Share the Blog, if you find it interesting and helpful.