why is my website showing The requested URL was not found on this server after uploading online code example

Example 1: wordpress inner page not found error

NOTE : If you got this error after installing wordpress on a newly configured 
ubuntu server
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1. sudo a2enmod rewrite

2. Add AllowOverride All in virtual host
<Directory /var/www/html>
    Options Indexes FollowSymLinks
    AllowOverride All
    Require all granted
</Directory>

3. restart apache

Example 2: wordpress main page is showing but other pages show page not found

There can be two solutions for this 
1. Modify .Htaccess with the code given below:

  # BEGIN WordPress

  RewriteEngine On
  RewriteBase /
  RewriteRule ^index.php$ - [L]
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  
  RewriteRule . /index.php [L] 
  #If project is inside the subfolder
  #RewriteRule . /PATH_TO_SUB_FOLDER/index.php [L] 

  # End WordPress
  
2. Correct The Permalinks Structure
	If you can access you admin panel, Then
      
      Step 1. Inside Settings and then on Permalinks
      (Take a look at the current setting. If you’re using a custom permalink
      structure, copy-save it somewhere.)
      
      Step 2. Select Default.
      
      Step 3. Click on Save Settings.

	If not then inside your database :
	
    options table => Search `permalink_structure` and set it to /%postname%/
    
reference: 
https://www.pickaweb.co.uk/kb/wordpress-fix-404-not-found-error/#:~:text=You%20can%20also%20fix%20the,permalinks%20of%20your%20WordPress%20site.&text=Log%20into%20WordPress%20Dashboard.,copy%2Dsave%20it%20somewhere.)