The commerce module has to be installed with Composer..The Commerceguys provide now an alternative. With Ludwig one can install Commerce without composer. The Ludwig module (Ludwig von Beethoven was a deaf composer --Drupal fun--) shows the manual way.
Putty is an open source Linux program for accessing the server. Something like Filezilla, but with Linux code. It requires SSH access. With Putty, you can use Composer. Composer is essential for Drupal 8.