Upgraded to Sierra and now Time Machine won't backup
It's been 18 days, so it's not an issue of the first backup being slow. The error message is that the backup disk could not be found. I have wifi though, so that's working. Any thoughts?
The error message is that the backup disk could not be found, which will occur if your network configuration has changed, or for a number of other reasons.
Nothing will happen until Time Machine can find that backup disk. To reconnect to the backup device, open Time Machine Preferences, and select "Add or remove backup disk". Select the backup disk, then Remove Disk and confirm that selection with Stop Using This Disk. Then, select "Add or remove backup disk" again. Re-select the same one under Available Disks, then Use Disk.
"Waiting to complete first backup" will appear, which means it won't do anything until the next scheduled backup. That does not mean it needs to create a completely new backup, as if it's creating a brand new one.
If you don't want to wait that long, select Back Up Now.
A Notification may appear if your source volume is encrypted and the backup volume is not. That's normal.
Your existing backups will remain unaffected, but since it has been more than a few days since the last backup, it will take a long time to finish that incremental backup. TM will perform a "deep scan" which always takes a long time. A few hours, perhaps. If you upgraded macOS at some time subsequent to the previous backup, three or four days might be necessary during which "Preparing Backup" will appear with no other apparent signs of progress. Let it finish.