Failed Catalina installation - not enough free space.

MacBook Pro, Late 2013, OS macOS Mojave 10.14.6. I attempt to upgrade to Catalina and got the failed installation message "There is not enough free space on the selected volume to upgrade the OS. An additional 854.7 MB is required." My About this Mac storage shows there is 13.33 GB available of 121.12 GB. I don't understand! What's going on? What should I do?