This is standard fare on High Sierra. Time Machine is simply not reliable.
Are you backing up to a local disk or a Time Capsule (TC) on the network?
If this is a time capsule.. I strongly recommend you swap to a 3rd party backup software like Carbon Copy Cloner.
If you want to continue to use Time Machine and you are currently on TC then a local disk plugged into the computer by USB or whatever other port is available depending on the age of the machine.
However if your currently on a local disk.. then issues can also arise to local drives.
Is the drive actually untitled?
Try giving it a name.. short no spaces and pure alphanumerics.
Reconfigure TM to the target with new name.
If you are really hopeful and the glass half full type.. with a TC.. then firstly try to verify the backup.
Hold down the option key and select verify. If TM cannot find the TC mount TC disk first in Finder and try again.
Most important you have no 3rd party anti-virus, additional security software or adware on the computer. It will cause issues to network drives.