Memory Warning When I have Pletny of Memory

I am trying to pack up an iphone 6s to my Mac. It has 32 GB storage. I hit the command to back up to my computer, it starts, but then a message pops up saying the backup could not be completed because there is not enough storage on the computer. I did a check of my available memory from "About this Mac," and it says there I have a 169 GB available out of a 1 TB. How do I solve this problem? Am I missing something?