It might be a long shot, but you could try erasing app data of the system update app:
Settings -> App Info -> 3 dots in the upper right -> Show System
Then scroll down to System Update -> Storage -> Clear Cache + Clear User Data
Since I'm not sure if this will work at all, you might wanna try a reboot afterwards, if you still can't access the system updater.
**this what being shared earlier by our friend above. Have you done all those steps?
Settings -> App Info -> 3 dots in the upper right -> Show System
Then scroll down to System Update -> Storage -> Clear Cache + Clear User Data
Since I'm not sure if this will work at all, you might wanna try a reboot afterwards, if you still can't access the system updater.
**this what being shared earlier by our friend above. Have you done all those steps?