Why can't I update Android when I removed the root? (Kingroot)

[TheCrazyBoy]

So... for a long time ago, I installed root because EVERYONE wanted me to install it because it was so cool and you could do many things with it.

Story behind the root:
I got Root access with Kingroot and while it was rooting my phone... it rebooted and I clicked the power button a few times and my Operative System was gone, the only thing that appeared was "Kernel is not seasonid enforcing". So I installed an android OS from a program and everything was fine, but I did a Factory Reset and removed all my data. And everytime I restart my phone and it boots, "Kernel is not seasonid enforcing" appears in a few seconds and then it boots into Android.

So after a year with root, I unrooted, and I think I did that wrong. First of all, I removed all the apps that was related with root and I thought... the root is removed! And I searched on the internet how to unroot and they said that I needed to use SuperSU, I did that and I think my root is gone but my OS Status says "Modified" so I can't update my phone anymore and I am not 100% if my phone is actually unrooted...

Information:
Android-Version: 5.1.1
Phone: Samsung Galaxy S6 - SM-G920F