@sdetweil followed the links and finally the npm install:
pi@raspberrypi:~/MagicMirror/modules/MMM-RAIN-RADAR $ npm install
up to date, audited 1 package in 2s
found 0 vulnerabilities
to Dennis
pi@raspberrypi:~/MagicMirror/modules/MMM-RAIN-RADAR $ npm install MMM-RAIN-RADAR
npm ERR! code E404
npm ERR! 404 Not Found - GET https://registry.npmjs.org/MMM-RAIN-RADAR - Not found
npm ERR! 404
npm ERR! 404 ‘MMM-RAIN-RADAR@*’ is not in this registry.
npm ERR! 404 This package name is not valid, because
npm ERR! 404 1. name can no longer contain capital letters
npm ERR! 404
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.
npm ERR! A complete log of this run can be found in:
npm ERR! /home/pi/.npm/_logs/2022-05-08T22_59_38_815Z-debug-0.log
pi@raspberrypi:~/MagicMirror/modules/MMM-RAIN-RADAR $ cd MMM-RAIN-RADAR