App Store won’t let me update from 10.13.4 to 10.13.6 - why? I’ve got 10.13.4 running on a MacBook Pro.

When I go to the App Store it says in order to download the latest version of iMovie I need 10.13.6. 

But there is no option to download 10.13.6 via the App Store.

Any ideas why this would be and apart from downloading the combo update file direct from Apple’s website (eg not through the App Store) how could I get it to work ?
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.