If you appear to have issue on uploading or deleting files to your server. Very likely you have issue with the folder permission issue.
Here an example if you have the same circumstance as mine.
If you are using Apache too, your group and user should default is www-data, so that the web services can access it. So, we first ensure the folders are in the same group and user
$sudo chown -R www-data:www-data /var/www
Then, secondly ensure your FTP access user is also inside the same group as www-data
$sudo useradd -g www-data <ftp_user>
you can use #id to check out the user group status
Then gives members in the group the permission to Write (g+w). Not understand? Can check this out
$sudo chmod -R g+w /var/www