MySQL 8.0

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    create table if not exists first(
    id int not null auto_increment primary key,
    Surname varchar(40) not null,
    Name varchar(40) not null,
    Patronymic varchar(40),
    Birth datetime not null default current_timestamp,
    phone varchar(20)
);
create table if not exists second(
contact_id int not null auto_increment primary key,
Person_ID int,
phone varchar(20),
mail varchar(30),
address varchar(40),
foreign key(Person_ID) references first(id),
 foreign key(phone) references first(phone)
);
Failed to add the foreign key constraint. Missing index for constraint 'second_ibfk_2' in the referenced table 'first'

CREATE INDEX first_phone ON first (phone);
create table if not exists second(
contact_id int not null auto_increment primary key,
Person_ID int,
phone varchar(20),
mail varchar(30),
address varchar(40),
foreign key(Person_ID) references first(id),
 foreign key(phone) references first(phone)
);