Magisk doesn't work on Android 4.x and earlier. I'm not familiar with the ROM you're using, but if it is Android 5.x or higher, Magisk should work. At the same time you're using an old version of Magisk: 17.3 was released yesterday.
If you want a more stock Android feel, you want to use Lineage OS as it is still being maintained. If by "stock" you mean Samsung's version of it, SuperWizCyan or Super Touchwiz are good choices, though the latter is no longer updated.
Thank you again :highfive:! My S4 has never looked so good, Lineage OS, Magisk and TWRP all installed and running like a dream (at this point).