.htaccess Redirect "/" to non "/" is 500 internal server error

I have an issue while google indexed my old URL where it doesn't longer exist, when someone clicked on the old link from Google search, there will shown 500 server error in my page.

My old URL is https://selfinjuryfoundation.org/idn-poker/ , which recently I was re-built a new rewrite rules in .htaccess to https://selfinjuryfoundation.org/idn-poker

The problem is "/"

How can I solve this issue?

My .htaccess

<IfModule mime_module>   AddHandler application/x-httpd-ea-php72 .php .php7 .phtml   ErrorDocument 404 https://selfinjuryfoundation.org   RewriteEngine on  RewriteCond %{THE_REQUEST} \.html [NC]  RewriteRule ^(.*)\.html$ /$1 [R=301,L]    RewriteCond %{REQUEST_FILENAME} !-d  RewriteCond %{REQUEST_FILENAME} !-f  RewriteCond %{REQUEST_FILENAME}\.html -f  RewriteRule ^(.*)$ $1.html [L]  </IfModule>  

https://stackoverflow.com/questions/65452805/htaccess-redirect-to-non-is-500-internal-server-error December 26, 2020 at 08:25AM