Creating a trigger but getting the unknown column error. Issues with if statement?

I'm trying to create a trigger that is fired after a row is inserted to update "m_name" to 'NOT GIVEN' if "m_name" IS NULL. But I get the error unknown column error. Also in my query, is the else statement necessary? I thought so since if m_name != null, keep m_name the same.Sorry pretty new to sql and any help is appreciated.