iOS 4.2.1 was released in November 2010. It was the last supported version for the and the iPod touch 2nd generation .
If prompted, select for the last compatible version. Sideloading via Jailbreak (Advanced): Viber .ipa for ios 4.2.1
: After jailbreaking (often using tools like Red Snow for iOS 4.2.1), you will need Cydia to install apps that manage .ipa installations. Viber .ipa for ios 4.2.1