Manually Update! Go to www.sammobile.com and download the exact software version of your mobile. Remember that you need to download according to your region and exact model number (example : international s9 plus is G965F) latest software. If your phone has not been updated since 2018, I think you might need to update gradually. I think s9 plus came with android oreo 8.0. Download Android 9 and install manually through Odin. If it turns out successfully try the auto software update afterwards, if it still doesn't work, try manual Android 10 installation then. Instructions are already provided. Carefully read them. Also it is totally your responsibility if you do it manually, if you end up messing your device, (hard brick) it's on you.
