[MENTION=3617]JAYZZY[/MENTION]
You're on official BTU firmware, don't think 6.0 is available for N910F yet. This is probably one of those pre-marshmallow updates with an updated bootloader to support the installation of the MM OTA when it comes.
You have 4 options:
1) Do a factory reset. Set up the device, don't install any apps, take the OTA and then continue.
2) Download Samsung Smart Manager. Connect your phone, ask it to check for updates and install through that.
3) You may have gotten a corrupt download. Find the directory where the OTA zip is stored, delete it. Reboot your phone and check for an update again, it should download it again. Do that with an uninterrupted internet connection and then install it.
4) Find the OTA directory, copy the zip file to root directory of the internal storage. Boot into stock recovery, press 'Apply update from zip' or 'Apply update from sdcard'. Select the file and install it. Proceed to do a cache wipe after you install. Then reboot.
NOTE: Don't know if this works. Flash at your own risk. You MAY or MAY NOT run into a bootloop.
Sent from my Nexus 6