[Rehan]

Chmod and chown should fix the problem . chown to www-data or add yourself to www-data group

If you use hosting you probably not encounter this issue but try chmod 777 the file that asked for permission . Because the cms in this case wordpress probably need the permission

The plugin installation problem has been solved by these commands.

chown www-data:www-data  -R * # Let Apache be owner
find . -type d -exec chmod 755 {} \;  # Change directory permissions rwxr-xr-x
find . -type f -exec chmod 644 {} \;  # Change file permissions rw-r--r--

But i still have the theme installation problem.I have changed the max file upload size via a plugin but that is useless.It is not working.Still searching for the solution.

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Setting permission through chmod is often not enough when working with web servers. For things to work properly you have to change ownership of all the files in /var/www to the user and group that is used to run the web server services. Usually (this highly depends on the OS though) the user and group is www-data (99% of all Debian and Ubuntu based distros have this user for that purpose).

So long story short: adjust file ownership also instead of only using chmod. To do that you can use the chown (change owner) command. Syntax: chown -R user:group /path/to/filesorfolders. -R stands for recursive to apply it to all sub folders and files. An example: chown -R www-data:www-data /var/www

Once that is done there is even no need for the dangerous chmod 777.

A lot of thanks.You are right the plugin installation problem has been fixed by chown.But still facing the theme installation problem.In past i fixed that theme installation problem by increasing the size from a file.I don't remember.But i am unable to find now.