App Store won’t let me update from 10.13.4 to 10.13.6 - why?
It is odd that the App Store app does not show you the update, not sure what is happening there. BUT You can manually download that update. The link is here.

That is what is referred to as a "Combo Update" which is a larger update that Apple provides that has the full size changed files rather than just a "delta" or changes. Combo updates are generally found to be a good repair for what ails macOS.