CPanel had discontinued SquirrelMail and will no longer be available on WHM version 78. Please see https://blog.cpanel.com/the-death-of-squirrelmail/ for more info.
Do not worry, no data will be lost and you can still make use of the more popular WebMail applications such as Roundcube and Horde.
While Squirrelmail can still be installed, we chose not to due to it being unintegrated with CPanel which will entail separate updates and may pose security issues.
Thank you for your understanding.
Friday, February 22, 2019