node-gyp (used to build better-sqlite3) is not compatible with Python 3. Yes, it's been 10 years since 3's release. Yes, it sucks. But you need to have python 2.7.x in your path to install better-sqlite3. If you're not using python anywhere else, uninstall it and make sure to follow my installation instructions. If you are, run
npm config set python python2.7 before running the install again.