The following assumes you’ve a complete and current backup on external storage. The following should not damage your files and your data, but there’s already something a little odd here, and your data is the hardest to recover if there’s something gone wrong.
Does shutting down and booting into Safe Mode allow the install? That rebuilds various caches, and also disables certain add-on software.
Are there any add-on anti-virus or anti-malware packages, any system cleaners or performance optimizer add-ons, or related packages installed here? Shut those down and remove them as a test, using the vendors’ provided removal documentation.
If Safe Mode and shutting off and removing any add-ons hasn’t helped, make a complete backup on external storage, then install the 10.13.6 combo update, then see if that allows subsequent updates to install. That combo update should not cause problems for what’s been installed nor,should it alter your own files, but can sometimes allow subsequent updates to be processed correctly.
