I'm running Drupal 8.1.5 site. At /admin/config/development/performance the "Bandwidth Optimization" settings for "Aggregate CSS Files" and "Aggregate Javascript files" are disabled, so I cannot change their state. Can you help me, please.
regards
Most likely you have these set as variables in settings.php or setting.local.php. Check those files for the appropriate variables (they should be obvious) and comment them out, or change them to your needs.
$config['system.performance']['css']['preprocess'] = TRUE; // or FALSE
$config['system.performance']['js']['preprocess'] = TRUE; // or FALSE
Make certain to flush the cache after making changes here.