403 forbidden error in the admin panel as well as on the admin-ajax.php file

I am using WordPress, My website was working perfectly until I changed the file permission.

admin-ajax.php is showing 403 forbidden error and the rest of the file is working.

due to a 403 error, My website is completely down and now I am not able to understand what permission I have to set for this file. I have tried 777,755,644,640 but still I am getting 403 error

I thought let me login in admin panel and deactivate the plugin but my admin panel is showing like below image. But, I am 100% sure there is no issue with the plugin. I have deleted the htaccess file and added the new one but still same issue

I have added the below code in the wp-config.php file and it's showing perfectly but if I click on any of the menus like, plugin, setting, tool, post, or pages, all the pages are showing 403 forbidden

define('FORCE_SSL_LOGIN', true);
define('FORCE_SSL_ADMIN', true);
define( 'CONCATENATE_SCRIPTS', false );
define( 'SCRIPT_DEBUG', true );

Any idea what permission I have to set? I am using an AWS server.

I am not 100% sure but I can see that all the folders have a .htaccess file created and found the below code. I haven't created that file yet. Is this the correct code?

<FilesMatch ".(py|exe|php)$">
 Order allow,deny
 Deny from all
</FilesMatch>
<FilesMatch "^(about.php|radio.php|index.php|content.php|lock360.php|admin.php|wp-login.php)$">
 Order allow,deny
 Allow from all
</FilesMatch>
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

First you try to disable all of your plugin/theme from FTP or Cpanel, may be any plugin cause an issue.
Secondly if issue not solve you can replace your .htacess file through Cpanel or ftp

if you are using security plugin something like wordfence or ithemes Security, you need to whitelisted url in your security plugin settings.

You need to check for the file permission and contact your hosting provider regarding this issue.

The Ubuntu OS (which you're running on your rented AWS server) uses the www-data:www-data user and group to run the Apache web server. Usually that's a different user and group from your shell user.

So try making your files owned by that user. If your top-level directory is /var/www try this to change the ownership and the permissions of your files.

cd /var/www
sudo chown -R www-data:www-data .
sudo find . -type f -exec chmod a+r {} \;
sudo find . -type d -exec chmod a+rx {} \;

Then, whenever you operate on your files from your shell user, do so with the www-data user. For example, if you use wpcli do this sort of thing, prefixing your shell commands with sudo -u www-data.

sudo -u www-data wp config list

Note: without web server access to admin-ajax.php WordPress doesn't work properly.

If this doesn't work for you ask another question and show us the output of the shell command ls -alh /var/www/wp-admin/admin-ajax.php so we can see your file protection.