As this was already the second time i faced the "Unable to install bootstrap, bootstrap.settings already exists in active configuration." problem i decided to share a quick and simple solution.
Background: I have faced this error when performing the Drupal 7 upgrade to the Drupal 9 version and trying to enable the bootstrap theme.
solution: in PHPMyAdmin open the database your Drupal site is running, in the Search tab enter "bootstrap"
