If your Normal, Search API Opensolr module won't work on your legacy Drupal, or any Drupal version, these steps might help.
Use the standard
Basic Auth instead of the
Official Opensolr Drupal Module, since that is no longer supported on Drupal 8,
however the Opensolr service is supported on any Solr and Drupal versions 
- Changed the index credentials to 123/123 (something simple), just to make sure all will work ok, using the
Opensolr Control Panel:
- Added the AUTH credentials in the Drupal admin:
- Added /solr to the Solr Path configuration in Drupal admin:
- Uploaded the Drupal 8 Solr version 3 Solr configuration files, using the Opensolr Configuration Control Panel. These are required in order for Drupal to identify the correct Solr index and version.
Check out the