Recently i ran into this error while trying to set up facets for my Drupal project and a product index. I decided to come up with a simplified solution for this in the following steps.
Step -1; This is my current content index
Step-2; When i click on the Index now button, i get an error as seen below
Step-3; Edit the index and uncheck the enable box and set the Server to No Server
Step-4; Index is now disabled
Step-5; Edit the Database server and delete All indexed Data
Step-6; Data successfully deleted
Step-7; Head back and re-enable your content index (In this case mine was a product index)
Step-8; You'll notice that the Track Now button has now re-appeared. Your items can now be re-tracked and then indexed later
Step-9; The next time you attempt to re-index your items, everything executes just fine😎
