Upgrading Drupal

We have a Drupal 8 site. I am relatively new to Drupal but learning fast! It is a composer install and the composer.json and the custom theme are in a github repository.

Hello everyone,

(Maintainence mode is on) running on Debian 10.

I ran composer update drupal/core "drupal/core-*" -- with-all-dependencies, and I got this error:

            :

            :

.... Update of composer/installers failed

     [RuntimeException] 

      Could not delete /var/www/html/xxxxx/vendor/composer/installers/composer.json:

I have composer.json and composer.lock on 775 mode at the domain level, not the core level.  However, for the /vendor/composer/composer.json file is currently on -rw-r--r--  root root Jan 14 2021

I'm getting errors updating a site with composer. I've updated a lot of times this site with a simple "composer update", but this time I'm getting a php error.

Curiously "composer update --dry-run" does not produce any error:

Can I upgrade from Drupal 6 to Drupal 9? If so, how?

As preparation for Drupal 9 I am trying to move onto Composer and am installing the Upgrade Status module:

https://www.drupal.org/project/upgrade_status

using the composer commands provided.  All commands run through without complaint but when I check /admin/reports there is no sign of an Upgrade Status report.  Am I missing something or is there a way to check if composer has actually installed the module?

Thanks

I am in the process of migrating a D8 site (8.9.18) to D9 (9.2.4).  I've been chasing down little problems, and am almost done with the migration, but one issue is stymying me: when creating or editing a node, the file picker in file_entity_browser that allows me to select an already-uploaded image doesn't work.  I can browse the files and inspect them.  But clicking to select a file has no effect.

Earlier in the migration, trying to open the file browser raised a white screen.  Inspecting the PHP error log after this showed this exception: