Creating a backup with adb results in 41 byte file

I have an old droid RAZR Maxx (4.1.2) that I decided I want to root, but I want to make a backup first just in case. I've followed instructions from this site:

How To Create a Full Android Phone or Tablet Backup Without Rooting or Unlocking Your Device

My command was this:

adb backup -apk -shared -all -f c:\backup.ab

The resulting backup.ab file on the C: drive was only 41 bytes and, when opened in a hex editor, appears to only be the header of the backup file. I'm wondering what I'm doing wrong here, I've googled but can't find anything that helps. Any suggestions?

Edit: This is not a duplicate of the suggested a topic linked by beeshyams. That topic explains why the user couldn't back up one single app's files, I'm trying to back up everything. I don't care if I don't get SMS included in that backup because it's an old phone, I'm not using it to text, but there are other apps and settings that should back up without issue.