503 Service Unavailable error was encountered

When I try to upgrade PHP to version 7.4 or 7.3, I get the following error:

Service Unavailable  The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.  Additionally, a 503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request. 

I tried to rename plugins folder and switched back to old PHP version and deactivated all pluigns, But still the same.

I activated twenty twenty-one and twenty twenty themes from WordPress, But still the same.

Finally I tried to logout and upgraded to PHP 7.4 and I could see the homepage. But when I try to visit any other page like /wp-admin or pages in the menu like /contact, I get the same error again.

I think it could be from .htaccess. Here is the code there:

RewriteCond %{HTTPS} off RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301] RewriteCond %{HTTP_HOST} !^www\. [NC] RewriteRule .* https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301] # BEGIN WordPress # The directives (lines) between "BEGIN WordPress" and "END WordPress" are # dynamically generated, and should only be modified via WordPress filters. # Any changes to the directives between these markers will be overwritten. <IfModule mod_rewrite.c> RewriteEngine On RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}] RewriteBase / RewriteRule ^index\.php$   - [L] RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule . /index.php [L] </IfModule>  # END WordPress      # SET CACHE <IfModule mod_rewrite.c>     <FilesMatch "\.(ico|pdf|flv|jpg|jpeg|png|gif|swf|eot|woff2|woff|ttf|svg)$  ">     Header set Cache-Control "max-age=31449600, public"     </FilesMatch>     <FilesMatch "\.(xml|txt|js|css|json)$  ">     Header set Cache-Control "max-age=604800, public, must-revalidate"     </FilesMatch>     <FilesMatch "\.(html|htm|php)$  ">     Header set Cache-Control "max-age=28800, must-revalidate"     </FilesMatch> </IfModule> # ZIP IMAGES <IfModule mod_deflate.c> SetOutputFilter DEFLATE BrowserMatch ^Mozilla/4 gzip-only-text/html BrowserMatch ^Mozilla/4\.0[678] no-gzip BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html SetEnvIfNoCase Request_URI \ \.(?:gif|jpe?g|png)$   no-gzip dont-vary Header append Vary User-Agent env=!dont-vary </IfModule>  # php -- BEGIN cPanel-generated handler, do not edit # Set the “ea-php74” package as the default “PHP” programming language. <IfModule mime_module>   AddHandler application/x-httpd-ea-php74 .php .php7 .phtml </IfModule> # php -- END cPanel-generated handler, do not edit 

I tried to comment the first part which redirects to https:

#RewriteCond %{HTTPS} off #RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301] #RewriteCond %{HTTP_HOST} !^www\. [NC] #RewriteRule .* https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301] 

And when I tried to visit http://mywebsite.com, I got that same error.

When I tried https://www.mywebsite.com, it loaded fine. But not other pages only homepage