Need Help in Htaccess

I can do coding very well but not much ideas with htaccess.

One of my client's website is created by other company but there is some issue as below.

https://www.amerrugs.com/about-amer
This url is working fine. But when adding slash / at last of URL like
https://www.amerrugs.com/about-amer/ then its giving internal error. While its working fine with product category means when we are opening category then its redirecting and adding slash (/) automatically https://www.amerrugs.com/power-loom/

This htaccess code is very new for me ...... So no ideas what i should add or change to fix this. htaccess code is as below

Options +FollowSymlinks

RewriteEngine on

RewriteCond %{HTTP_HOST} !^www\. [NC]

RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/cpanel-dcv/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/(?:\ Ballot169)?
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/[A-F0-9]{32}\.txt(?:\ Comodo\ DCV)?$
RewriteRule ^(.*)$ http://localhost/amerrugs/$1 [L,R=301]



RewriteCond %{HTTPS} on

RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/cpanel-dcv/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/(?:\ Ballot169)?
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/[A-F0-9]{32}\.txt(?:\ Comodo\ DCV)?$
RewriteRule ^(.*)$ http://localhost/amerrugs/$1 [L,R=301]

ErrorDocument 404 /404.php



RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/cpanel-dcv/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/(?:\ Ballot169)?
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/[A-F0-9]{32}\.txt(?:\ Comodo\ DCV)?$
RewriteRule ^(.*)index\.(php|html?)$ /$1 [R=301,NC,L]




RewriteCond %{REQUEST_FILENAME} !-d

RewriteCond %{REQUEST_FILENAME}.php -f

RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/cpanel-dcv/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/(?:\ Ballot169)?
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/[A-F0-9]{32}\.txt(?:\ Comodo\ DCV)?$
RewriteRule ^(.*)$ $1.php



# Return 404 if original request is .php

RewriteCond %{THE_REQUEST} "^[^ ]* .*?\.php[? ].*$"

RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/cpanel-dcv/[0-9a-zA-Z_-]+$
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/(?:\ Ballot169)?
RewriteCond %{REQUEST_URI} !^/\.well-known/pki-validation/[A-F0-9]{32}\.txt(?:\ Comodo\ DCV)?$
RewriteRule .* - [L,R=404]

Code (markup):

Need help of any htaccess expert to fix issue of this website.

Thanks
Suraj Anjaana