Error: pq: at or near "update_assignee_columns_trigger": syntax error: unimplemented: this syntax

Question

I want to create trigger in postgresql.

Using trigger, I want functionality like if tasks table's assignee_individual field is not null then assignee_team field would be null default.

I have written commands for trigger but it gives me error, at or near "update_assignee_columns_trigger": syntax error: unimplemented: this syntax

CREATE FUNCTION update_assignee_columns()
RETURNS TRIGGER AS $$
BEGIN
IF NEW.assignee_individual IS NOT NULL THEN
NEW.assignee_team = NULL;
END IF;

IF NEW.assignee_team IS NOT NULL THEN
NEW.assignee_individual = NULL;
END IF;

RETURN NEW;
END;
LANGUAGE plpgsql;

CREATE TRIGGER update_assignee_columns_trigger BEFORE INSERT OR UPDATE ON tasks FOR EACH ROW
EXECUTE PROCEDURE update_assignee_columns();