Fixing the "User warning: The following module is missing from the file system" error in Drupal 8
When you get this error, it means you removed or moved module files without un-installing it first. Luckily, there is an easy fix. There is an entry in the
key_value table that needs to be deleted. You can do this with drush (replace module_name with the name of your module):
drush sql-query "DELETE FROM key_value WHERE collection='system.schema' AND name='module_name';"
Be sure to rebuild caches afterwards:
For more details, check out this help page: https://www.drupal.org/node/2487215.