Failed to add the foreign key constraint. Missing index for constraint ‘information_ibfk_1’ in the referenced table ‘informacia’

What is the problem with these tables?

create table informacia
(
    adress_id int primary key,
    street varchar(20),
    city varchar(10),
    state varchar(2),
    zip_code int
);

I want to create foreign key for second one

create table information
(
    FirstName varchar(10) ,
    LastName varchar(15),
    AdressLine1 varchar(20),
    City varchar(10),
    StateProvinceCode varchar(2),
    PostalCode int,

    foreign key(PostalCode) 
         references informacia(zip_code) on delete set null
);

Answer

In order for a table to add a foreign key constraint, the referenced column (zip_code in this case) must be a PRIMARY KEY or must have a UNIQUE constraint (ideally with NOT NULL).

I modified the SQL statements according to PostgreSQL database. You don’t mention the specific database and PostgreSQL implements a highly standard SQL. The script, modified with two changes, is:

create table informacia(
    adress_id int primary key,
    street varchar(20),
    city varchar(10),
    state varchar(2),
    zip_code int not null, -- added NOT NULL
    constraint uq_zip_code unique (zip_code) -- added the UNIQUE constranit
);

create table information(
    FirstName varchar(10) ,
    LastName varchar(15),
    AdressLine1 varchar(20),
    City varchar(10),
    StateProvinceCode varchar(2),
    PostalCode int,
    foreign key(PostalCode) references informacia(zip_code) on delete set NULL
);

See running example at DB Fiddle.