PostgreSQL "type serial does not exist"

export const users = pgTable(
  "users",
  {
    id: serial('id').primaryKey(),
    clerk_id: varchar("user_id", { length: 50 }),
    username: varchar("username", { length: 100 }),
  },
  (users) => {
    return {
      username_idx: uniqueIndex("username_idx").on(users.username),
    };
  }
);

#this is the error displayed when try to push changes
error: type "serial" does not exist