Unable to run update.php or update modules from web interface
update.php does not update modules. It updates the database state if needed (after installing module updates, aka db-update). 

Did you check the permissions of your /modules/contrib/* filesystem? When you update your modules with the admin UI, your webserver's user (apache or www-run...) needs to have write permission there.

Did you install your instance with composer? If so, do not use the admin UI.

Maybe, check the docs again: https://www.drupal.org/docs/user_guide/en/security-update-module.html