SquirrelMail has been removed from cPanel, as of version 78. SquirrelMail is no longer maintained (its last update was in 2013) and it had a security flaw that required its removal. There are two alternative webmail packages that can be used instead: Horde or RoundCube. Both are excellent choices. We recommend Horde if you sometimes check mail from your phone or tablet. If you require HTML email, we recommend RoundCube.
If you need your address book exported from SquirrelMail over to Horde or RoundCube, please open a support ticket and our staff will be happy to move it over for you. Be sure to include the email address and your choice of Horde or RoundCube.