As per recommendation of GTMetrix, I added the following code into .htaccess but it has no effect at all.

<IfModule mod_expires.c>
# Enable expirations
ExpiresActive On 
# Default directive
ExpiresDefault "access plus 1 month"
# My favicon
ExpiresByType image/x-icon "access plus 1 year"
# Images
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpg "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
# CSS
ExpiresByType text/css "access 1 month"
# Javascript
ExpiresByType application/javascript "access plus 1 year"
</IfModule>

I tried inserting the above code into /etc/apache2/apache2.conf, but no luck.
Somebody help me please.

Do you have the module expires enabled?

I dont know how to enable it.

I got it.
after executing the following command.

sudo a2enmod expires

Thanks for the help.