The IMAP extension is missing from your PHP installation.
In case this kind of error isn’t pretty self explanatory, your php instance doesn’t contain the imap extension.
If you have access to command line, following commands should install php imap on your server:
If you are on CentOS / RedHat / Fedora or derivatives:
yum install php-imap
If you are on a Debian / Ubuntu or derivatives:
apt-get install php-imap
If no packages are found, then consult your distro manual.
In case you are on a host where you don’t have command line access, if you have CPanel/WHM access, you can use EasyApache to compile php with imap extension.
Otherwise, you have to contact your host about the issue.