Your server needs to have the PHP OpenSSL extension installed and enabled. Moreover, the folder administrator/components/com_akeebabackup/engine needs to be writeable by the user your site is running under.
If both conditions are met and you still see this message I believe you are also seeing a Joomla warning about File: Infinite Loop Detected. This is a bug in Joomla itself which prevents the administrator/components/com_akeebabackup/engine/serverkey.php file (containing a settings encryption key created at first run and when the file is missing using a cryptographically secure pseudorandom generator) from being written to. We will make a new release a bit later today where we are no longer using Joomla's buggy filesystem API, instead reverting to only using the built-in PHP functions for file access, overcoming this problem.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!