[Plugin: AskApache Google 404] plugin giving errors on the 404 page

I am currently getting weird errors on this site: https://www.die-kellerkinder.eu just add anything you want to the end of the url to see the errors, i.e. https://www.die-kellerkinder.eu/ffun

here is the error:

Related Pages to the Request

WordPress database error: [Can’t find FULLTEXT index matching the column list]
SELECT ID, post_title, post_content,MATCH (post_name, post_content) AGAINST (‘ffun’) AS score FROM wpwp_posts WHERE MATCH (post_name, post_content) AGAINST (‘ffun’) AND post_date <= ‘2008-09-06 11:17:40’ AND (post_status IN ( ‘publish’, ‘static’ )) AND post_password =” ORDER BY score DESC LIMIT 2

and below it:

More Related Pages to the Request

WordPress database error: [Can’t find FULLTEXT index matching the column list]
SELECT ID, post_title, post_content,MATCH (post_name, post_content) AGAINST (‘ffun’) AS score FROM wpwp_posts WHERE MATCH (post_name, post_content) AGAINST (‘ffun’) AND post_date <= ‘2008-09-06 11:17:40’ AND (post_status IN ( ‘publish’, ‘static’ )) AND post_password =” ORDER BY score DESC LIMIT 6

https://www.ads-software.com/extend/plugins/askapache-google-404/