Updating drupal/core using composer: "[RuntimeException] Could not delete web/core" (using Lando)
OK - so composer does not have read/write access to the folder. I've also seen some people mentioning composer timeouts being the problem.
In my case I am using Lando for my dev environment ands listed web/core as one of the excludes:
excludes:
  - web/core

This meant the core files were not available to the container. If I remove core from the excludes then it works fine.