iPhone Backup Could Not Be Completed

Phone is an iPhone 8; computer is a MacBook Pro running High Sierra. Attempting to back up the iPhone to my computer (not to iCloud) using iTunes, and I keep getting an error saying the backup could not be completed because there isn't enough free space on my computer. I have 300 GB of free storage and the phone's capacity is 64 GB (and it isn't nearly full). I've tried rebooting both the phone and the laptop. There were no previous backups for this phone (it's new) but I deleted an old backup of an older phone with the same phone name. There are no other backups of that phone. iOS on the phone is up to date. How can I resolve this?