Have to unlock bootloader first then install magisk. See developer threads for bootloader unlock service only if it's on bootloader version 1. To check bootloader version it look in settings/about/software the number 5th from the right is the bootloader version. Your 991w should be a snapdragon processor. The exynos model is 991F and you can unlock bootloader for free you can get oem unlock to show up by following guides on google.