What version of Roundcube are you installing?According to some old Debian bugs it looks like the php5-intl package would handle the Net_IDNA2 class.
Why you're installing beta if there are stable releases for this branch? Anyway, you need Net_IDNA2 which is a PEAR library or php-intl extension. Why don't you use composer?
When I try to install Roundcube, it shows that I have a single error. It says, "Net_IDNA2: NOT OK(Failed to load class Net_IDNA2 from pear.php.net; See http://pear.php.net/package/Net_IDNA2)"All my PHP and libraries seem to be installed. Below is a screen shot. Currently running Ubuntu 16.04 / Nginx.Do you know how I can fix this?