The option to install Roundcube will be visible only after creating the website in your server.

For this, click on the Website name as per the image below:

Then click on the option Enable Webmail as per the screenshot below:

Then you need to enter the subdomain you would like to use for the webmail/Roundcube. Let say, if you are going to install Roundcube on rc.example.com then you need to enter only rc on the Domain name field and choose CloudFlare account where your domain is added and click on the button Install Roundcube.

Then this will install Roundcube and SSL for webmail automatically.

Incase SSL installation fail, you need to click on the option Manage SSL >> Deploy New SSL >> Select the Roundcube application. Choose the authorization medthod as HTTP or DNS and click on Only HTTPS Then, click on the button Deploy SSL.