Download the combo updater and run it over your install. If that fails, you can download the full Sierra 10.12.6 installer and run it over your data.
Download macOS Sierra 10.12.6 Combo Update
https://support.apple.com/kb/DL1931
Post Date: Jul 19, 2017 File Size: 1.98 GB
MORE INFO ON WHY RUNNING COMBO FIXES ISSUES
Apple updates available from the Software Update application are incremental updates. Delta updates are also incremental updates and are available from Apple Downloads (software updates are generally smaller than delta updates). The Combo updates contain all incremental updates and will update files that could have become corrupted.
Combo updaters will install on the same version as they're applying--no need to roll back or do a clean install.
"Delta" updaters can only take you from one version to the next. For example: 10.12.5 to 10.12.6. If somehow the 10.12.5 is missing something it should have, and that something isn't changed between 10.12.5 and 10.12.6 it will still be stale after the delta update.
Both options are considered safe, but as usual, you should always backup your drive first with Time Machine and clone backups.