No, you can’t have a single cory catfish. Cory catfish are schooling fish and thus, feel happier with other corys around. A single cory catfish will feel lonely and stressed. Eventually, the lonely cory might refuse to eat, fall ill, and die.
You should keep cory catfish in a group of six or more. At the very least, you should keep a pair of cory catfish. Make sure they belong to the same species, otherwise they won’t properly form a school and will still feel lonely.