All of the previous answers are correct, but they did not work for me. The only way I was able to make this work was by the following steps/commands:
npm uninstall -g @angular/[email protected]
npm cache clean --force
npm install -g @angular/cli@latest
npm install node-sass -g
ng new MY_PROJECT_NAME
cp -r from_my_old_project to_new_MY_PROJECT_NAME