Autocomplete not working for Search API with Solr - Drupal 7
The README.txt file in the search_api_autocomplete module provides the following information:


Necessary setup


After having installed and enabled the module, you have to do some
administrative steps to activate the autocomplete functionality. Autocompletion
can be enabled and configured for each search separately.

To activate autocompletion for an index's searches, go to the index's
„Autocomplete“ tab. There, you see all available searches for the index and can
enable (and afterwards configure) autocompletion for each of them. All fulltext
key fields on the searches should then become autocompletion fields.