Ban users by IP giving error

Hey,
first of all great plugin thanks!

I’m trying to ban a certain country from accessing my site. I’ve pasted the relevant IP addresses but get the following error:
101.0.4.0/255.255.252.0 is not a valid address as it has been white listed.

I get this on almost every IP address I post except 10.
Any idea why ?

https://wordpress.org/plugins/better-wp-security/