Drupal 8 & Solr 8.1: No search results, error: "The datasource with ID '' could not be retrieved for index..."
Possible problem: "Search API Solr" module 8.x-2.7 does not support Solr 8.

Solution:


Remove "Search API Solr" module 8.x-2.7 using 'composer remove drupal/search_api_solr'
Install "Search API Solr" module 8.x-3.1 using 'composer require drupal/search_api_solr'
Find Solr modules in Drupal BO ("Extend" > "List") and click "Install" button
Add Solr Server in Drupal BO. I used solr path "/". 
Download the configs zip folder from Drupal BO, copy files from the folder to the conf directory of the Solr core. (See details in INSTALL.md file in the module directory).
Restart solr server.
In Drupal BO in my Index configs select the name of my Solr server.
Go to the Index in Drupal BO. Click to run the index of content.
Go to my previously confiured view. See search results and search!