If you are free users, you can only download podcasts. The ability to download music offline on Spotify for desktop is only available to Premium members.
Here's everything you need to know about downloading music from Spotify for offline listening on desktop. FAQs about 'Download Spotify Music to Computer' Everything You Should Know about Spotify for Desktop Record and Download Music from Spotify to Computer Online Part 5.
Part 1: Download Playlist & Album on Spotify Desktop with Premium Part 2: Download Spotify Podcast to Computer with Free/Premium Account Part 3: Download Music from Spotify Computer without Premium (Best & Free) Part 4.