Local Files never sync to phone

[charlie97]

Plan

Free/Premium

Country

USA 

Device

iPhone 7 & Windows PC

Operating System

iOS 10

My Question or Issue

 I have local files listed on my desktop version of spotify. I try to play them and my phone says first I have to put them in a playlist and mark it to sync. Very vague instructions on how to mark a playlist like this.

I just want the local music on my computer to transfer to my phone, it has literally been years and I haven't ever been able to fix the problem. Why won't this work???