ykanazawa

YARPP requires fulltext index.
YARPP works only with MyISAM, not works with InnoDB, Because fulltext index only works with MyISAM.
However, recently, fulltext index now works in InnoDB.(MySQL5.6.4 or higher)
But, YARPP still does not support InnoDB Fulltext Index.

If you can use MySQL CLI, I think that you can change it directly to MyISAM by MySQL CLI.

I fixed it so that InnoDB Fulltext Index can be used.
https://plugins.trac.www.ads-software.com/ticket/2655
I hope that this will be adopted.

Reference:
Support for InnoDB with full-text index
https://www.ads-software.com/support/topic/support-for-with-full-text-index/
Modify table to MyISAM, Try create full indexes and fail
https://www.ads-software.com/support/topic/modify-table-to-myisam-try-create-full-indexes-and-fail/

Looking at this,
https://plugins.trac.www.ads-software.com/browser/yet-another-related-posts-plugin/trunk/includes/yarpp_myisam_notice.php

I think the check of myisam and fulltext may not be perfect.

If you can use mysql CLI, It is also possible by the following method.

mysql> DROP INDEX yarpp_title ON dme3_posts;
mysql> DROP INDEX yarpp_content ON dme3_posts;

mysql> ALTER TABLE dme3_posts ADD FULLTEXT INDEX yarpp_title(post_title);
mysql> ALTER TABLE dme3_posts ADD FULLTEXT INDEX yarpp_content(post_content);