Why can't i install software upgrade on galaxy s6

[Zubair_14]

"THE OPERATING SYSTEM ON YOUR DEVICE HAS BEEN MODIFIED IN AN UNAUTHORISED WAY. pLEASE TRY............"
My Dad received his new samsung galaxy s6 that we bought from an online store ( "Souq.com ) 5 days ago. The box came sealed and had all the things fine. After my dad used it for a day i realised that the OS was Lollipop but i knew that s6 can be marshmallow so i went to settings to check for an update and this message cam up (first line) We didnt root the device or anything. We transferred his things using smart switch ( dont think that could be the cause though). Please help me as soon as possible. Thank You

 

[mhinc]

Hey. That message only comes up if your phone has been rooted and a custom OS was installed.
The message is there to prevent issues of you installing an official OS update over top of a custom ROM,
So long story short, your phone is Rooted.