First thing you need to do is take a backup of your current state. Then we start debugging.
A lot of things could cause this error so we have to first determine the root cause. To do this go to your wp-admin folder and in the wp-config.php file add the following line of code:
define( 'WP_DEBUG', true );
This will allow you to see the error that is on each page. Be sure to remove the code when you're done.
When we know the error or the cause of the error it will be easier to solve. Some simple things you can start with is turning off all plugins, reverting to default theme, etc.
Please see if the url used accessing the page is right. Please refer the link for creating plugin menu pages https://developer.wordpress.org/reference/functions/add_menu_page/ and https://developer.wordpress.org/reference/functions/add_submenu_page/