The host has a bad configuration - the temp directory it is exposing is not accessible. But with a few steps you can create your own.
Create a folder using FTP that is not visible on your website, so next to public_html instead of inside it, if you have that folder. Or create the folder anywhere and use your control panel to password protect it.
Then go to General Settings and look at the text next to Filesystem excludes, it will say something like this:
Custom filesystem excludes can be specified here, one per line, relative to the following folder: /var/www/homes/user/public_html
This is the absolute path to your WordPress website. You can use this to work out the absolute path to the folder you just created as you will need it.
Next, look in the address bar of your browser where it says: wp-admin/tools.php?page=wponlinebackup%2Fwponlinebackup.php§ion=settings
Change the end to advanced, so it says: wp-admin/tools.php?page=wponlinebackup%2Fwponlinebackup.php§ion=advanced
This will load the hidden advanced settings page - be careful not to change anything here unless you understand it as it can break the plugin, thus why it is hidden.
On this page, look for gzip_tmp_dir and tick the checkbox and change the path to the folder you created - it needs to be absolute path and you need to tick the box or it will ignore the new value - ticking it means to override it. Click Save and it should apply.
Try running a backup then.