boot your phone into fastboot (hold vol down and Power button)
open cmd and type in
fastboot boot twrp-3.7.1_12-v9.0_A15-odin-skkk.img
In TWRP, go on Advanced and then on Install Recovery Ramdisk
reboot phone into recovery again.
when you have the ROM on your internal storage, go on install, select the ROM zip and continue.
If you had root with Magisk before, you have to do the root process again.
just download Magisk and flash the Magisk apk in recovery just like you installed the ROM.