Why doesn't changing the table work?

Question

PostgreSQL

Why does not work like everything is done correctly. The table has already been created. This is just an example

create table persons(
rols varchar(30) not null
);

alter table persons 
alter column rols
add check (rols in('registrator', 'coordinator', 'user'))

Before check, the error says
LINE 3: add check (rols in('registrator', 'coordinator', 'user'))

Answer 1

Because there is no such alter column syntax.

Answer 2

Melkij , the case speaks)
see PostgreSQL CHECK Constraint

ALTER TABLE persons 
   ADD CONSTRAINT persons_check_rols CHECK (rols in('registrator', 'coordinator', 'user'));