Volker can tell me I'm wrong, but I don't think table CHECK constraints can include cross-table queries.

In your case, each of your constraints may require two TRIGGER statements to handle both INSERT and UPDATE statements on two tables. The following code shows one pair of triggers.

NOTE: Only one error condition was tested.

BEGIN
   DROP TABLE hotels;
   EXCEPTION WHEN OTHERS THEN
END;
BEGIN
   DROP TABLE clients;
   EXCEPTION WHEN OTHERS THEN
END;
BEGIN
   DROP TABLE orders;
   EXCEPTION WHEN OTHERS THEN
END;
BEGIN
   DROP TABLE rooms;
   EXCEPTION WHEN OTHERS THEN
END;
BEGIN
   DROP TABLE hotels;
   EXCEPTION WHEN OTHERS THEN
END;
BEGIN
   DROP TABLE rezervation;
   EXCEPTION WHEN OTHERS THEN
END;

CREATE TABLE hotels(
id INTEGER PRIMARY KEY, 
name VARCHAR ( 50 ));

CREATE TABLE clients(
passport_number INTEGER PRIMARY KEY, 
name VARCHAR ( 50 ), 
date_of_birth DATE ); 

CREATE TABLE orders(
id INTEGER PRIMARY KEY, 
orders_date DATE, 
clients_passport_number INTEGER REFERENCES clients ( passport_number ),
hotels_id INTEGER REFERENCES hotels ( id ) );

CREATE TABLE rooms(
number_of_room INTEGER PRIMARY KEY, 
price DECIMAL ( 9, 2 ), 
hotels_id INTEGER REFERENCES hotels ( id ) );

CREATE TABLE rezervation(
rooms_number_of_room INTEGER REFERENCES rooms ( number_of_room ), 
orders_id INTEGER REFERENCES orders ( id ),
reservation_from DATE, 
reservation_to DATE );

CREATE TRIGGER orders1 BEFORE INSERT, UPDATE ON orders 
REFERENCING NEW AS new_orders FOR EACH ROW
BEGIN
   IF EXISTS ( SELECT * 
                 FROM rezervation 
                WHERE new_orders.orders_date > rezervation.reservation_from 
                  AND new_orders.id          = rezervation.orders_id ) THEN
       ROLLBACK TRIGGER WITH RAISERROR 99999 
          'orders_date should be earlier than reservation_from';
   END IF;
END;

CREATE TRIGGER rezervation1 BEFORE INSERT, UPDATE ON rezervation 
REFERENCING NEW AS new_rezervation FOR EACH ROW 
BEGIN
   IF EXISTS ( SELECT * FROM orders 
                WHERE orders.orders_date > new_rezervation.reservation_from 
                  AND orders.id          = new_rezervation.orders_id ) THEN
       ROLLBACK TRIGGER WITH RAISERROR 99999 
          'orders_date should be earlier than reservation_from';
   END IF;
END;

INSERT hotels VALUES ( 111, 'xxx' );
INSERT clients VALUES ( 999, 'aaa', '1901-01-01' );
INSERT orders VALUES ( 222, '2020-05-03', 999, 111 );
INSERT rooms VALUES ( 666, 200.00, 111 );
INSERT rezervation VALUES ( 666, 222, '2020-05-04', '2020-05-05' );

INSERT orders VALUES ( 223, '2020-05-11', 999, 111 );
INSERT rezervation VALUES ( 666, 223, '2020-05-10', '2020-05-12' );

Could not execute statement.
RAISERROR executed: orders_date should be earlier than reservation_from
SQLCODE=-99999, ODBC 3 State="HY000"
Line 1, column 1
INSERT rezervation VALUES ( 666, 223, '2020-05-10', '2020-05-12' )