Can't access specific admin pages
Not sure whether or not this will actually fix your issue, but I'd try:


update the drupal/core version to in composer.json to be ^8.7.0
if you're using webflo/drupal-core-require-dev, update that to ^8.7.0 as per https://www.drupal.org/docs/8/update/update-core-via-composer
run composer update drupal/core webflo/drupal-core-require-dev --with-dependencies
run your database updates, drush updb or site.com/update.php


If that still doesn't work, try:


deleting your vendor directory, core directory, and composer.lock
run composer install