15.5 GB is insufficient free space. Even with 128 GB, you need a bare minimum of 20 GB free to install it.
Turn off Time Machine once your backup is complete, and then try to do the combo update.
Backup any data that you don't use frequently to two other hard drives and/or flash drives, or use Carbonite or Cloud backup options such as iCloud, OneDrive, or Google Drive.
A general rule of thumb, though not documented anywhere as a manufacturer requirement is to keep at least 15% of your disk space free for swap files and temporary files not to get in the way of installs.
That basically means:
20 GB for 128 GB,
40 GB for 256 GB
80 GB for 512 GB
160 GB for 1 TB.
It will also keep your system running smoothly.
Only upgrade when you have a genuine need to run something that won't run any earlier.