ssh to your server and check if the permissions of /tmp are set to 777. If not, the cpanel version of php can not be executed. Chmod /tmp to 777.
-- If this doesn't help, do a
if the results do not contain
then you do not have the cpanel version of php installed. Perform a cpanel update to fix (a cpanel update can be done using WHM).
-- Also make sure that the Fantastico admin account has enough free space (quota). This should be better set to "unlimited" anyway.
-- Also check if your hard disk is full or if the /tmp partition is full.