What specifically does it say when it goes to upload mode? Does it say "Unknow Error"? "Modem Crash"? "User pressed power key for 7 seconds?" Look at the small text (if there is any), does it say something about "ext4"?
Upload mode is just the default state a phone running the factory firmware goes to when it cant continue running, where if you were running stock it would just hard reset. There are tons of different reasons for why this could happen: Linux kernel panic, Modem crashed, Cant mount root filesystem, cant find init, something very low level in the device crashed, etc etc etc. In most cases, it will tell you somewhere on the upload mode screen at least what general thing caused the crash, and maybe more specifics if its one of a slightly less number of possible causes.
And since you are new, welcome to the world of Locked bootloaders and root! It is tricky because we are doing things that Samsung spent a lot of time, money and engineering effort specifically to stop us from doing. In all honesty next device around if you want something that is much easier and more "turnkey" than this, the only option for Samsung is to pick up one of the exynos devices from europe. They don't support CDMA carriers (there arent any in Europe), but they have bootloaders that you can unlock and then its open season, you can flash whatevever you want without Auth errors and Invalid Arguments and all the stuff we deal with here.
It says Kernel Panic! Then proceeds with this:
PC is at sysrq_handle_crash+0x20/0x2c
LR is at sysrq_handle_crash+0xc/0x2c
Panic caller: die+0x28c/0x2d0
Panic Msg : Fatal exception
Thread : init:499
last_ms : 0x24
Additionally, Windows 10 has decided that my s8+ is an unregonizable device. Which may have just become the primary problem which solving will actually solve nothing. Sigh. Anyone?
Reinstalled Samsung USB drivers and phone is back on good terms with Windows 10. Still.. upload mode?