Data Warehouse error related to a group by mistake - oracle

Question

So I have 2 tables:Clients_Test and Vacations_Test.I need to create a warehouse that has 2 dimensions:
d1_month and d2_destination.The main table that shoud contain the fields of these 2 dimensions is the fact table.Also the fact needs to have 2 more fields:the total destinations of each client + the total price.
Problem is after I insert data into the fact table the group by line breaks down and I don't know why.Tried to group them using other fields but still it keeps me back.
CREATE TABLE CLIENTS_TEST(
IDclient NUMBER(7) PRIMARY KEY,
name_client VARCHAR2(40) ,
adress VARCHAR2(50),
city VARCHAR2(30),
county VARCHAR2(20));
CREATE TABLE VACATIONS_TEST(
number_contract NUMBER(6) PRIMARY KEY,
IDclient NUMBER(7) REFERENCES CLIENTI_TEST(IDclient),
date_contract DATE,
destination VARCHAR2(40),
price NUMBER(10));
INSERT INTO CLIENTS_TEST VALUES (1, 'name1','adr1', 'Timisoara', 'Timis');
INSERT INTO CLIENTS_TEST VALUES (2, 'name2','adr2', 'Arad', 'Arad');
INSERT INTO CLIENTS_TEST VALUES (3, 'name3','adr3', 'Cluj', 'Cluj');
INSERT INTO CLIENTS_TEST VALUES (4, 'name4','adr4', 'Arad', 'Arad');
INSERT INTO CLIENTS_TEST VALUES (5, 'name5','adr5', 'Timisoara', 'Timis');
INSERT INTO CLIENTS_TEST VALUES (6, 'name6','adr6', 'Cluj', 'Cluj');
INSERT INTO VACATIONS_TEST VALUES (11, 2, TO_DATE('2/05/2020','dd/mm/yyyy'), 'dest2', 4000);
INSERT INTO VACATIONS_TEST VALUES (12, 3, TO_DATE('4/05/2020','dd/mm/yyyy'), 'dest5', 4000);
INSERT INTO VACATIONS_TEST VALUES (13, 6, TO_DATE('8/05/2020','dd/mm/yyyy'), 'dest3', 3000);
INSERT INTO VACATIONS_TEST VALUES (14, 1, TO_DATE('10/05/2020','dd/mm/yyyy'), 'dest4', 3000);
INSERT INTO VACATIONS_TEST VALUES (15, 3, TO_DATE('12/05/2020','dd/mm/yyyy'), 'dest1', 2000);
INSERT INTO VACATIONS_TEST VALUES (16, 5, TO_DATE('15/05/2020','dd/mm/yyyy'), 'dest2', 4000);
INSERT INTO VACATIONS_TEST VALUES (17, 2, TO_DATE('18/05/2020','dd/mm/yyyy'), 'dest4', 5000);
INSERT INTO VACATIONS_TEST VALUES (18, 6, TO_DATE('21/05/2020','dd/mm/yyyy'), 'dest5', 3000);
INSERT INTO VACATIONS_TEST VALUES (19, 1, TO_DATE('24/05/2020','dd/mm/yyyy'), 'dest1', 4000);
INSERT INTO VACATIONS_TEST VALUES (20, 4, TO_DATE('27/05/2020','dd/mm/yyyy'), 'dest3', 6000);
INSERT INTO VACATIONS_TEST VALUES (21, 3, TO_DATE('3/06/2020','dd/mm/yyyy'), 'dest5', 4000);
INSERT INTO VACATIONS_TEST VALUES (22, 4, TO_DATE('6/06/2020','dd/mm/yyyy'), 'dest2', 3000);
INSERT INTO VACATIONS_TEST VALUES (23, 6, TO_DATE('7/06/2020','dd/mm/yyyy'), 'dest4', 6000);
INSERT INTO VACATIONS_TEST VALUES (24, 2, TO_DATE('9/06/2020','dd/mm/yyyy'), 'dest5', 5000);
INSERT INTO VACATIONS_TEST VALUES (25, 5, TO_DATE('11/06/2020','dd/mm/yyyy'), 'dest2', 4000);
INSERT INTO VACATIONS_TEST VALUES (26, 3, TO_DATE('14/06/2020','dd/mm/yyyy'), 'dest1', 3000);
INSERT INTO VACATIONS_TEST VALUES (27, 2, TO_DATE('17/06/2020','dd/mm/yyyy'), 'dest4', 6000);
INSERT INTO VACATIONS_TEST VALUES (28, 6, TO_DATE('19/06/2020','dd/mm/yyyy'), 'dest5', 4000);
INSERT INTO VACATIONS_TEST VALUES (29, 1, TO_DATE('21/06/2020','dd/mm/yyyy'), 'dest3', 2000);
INSERT INTO VACATIONS_TEST VALUES (30, 5, TO_DATE('27/06/2020','dd/mm/yyyy'), 'dest4', 3000);
INSERT INTO VACATIONS_TEST VALUES (31, 4, TO_DATE('1/07/2020','dd/mm/yyyy'), 'dest5', 3000);
INSERT INTO VACATIONS_TEST VALUES (32, 3, TO_DATE('4/07/2020','dd/mm/yyyy'), 'dest1', 2000);
INSERT INTO VACATIONS_TEST VALUES (33, 1, TO_DATE('6/07/2020','dd/mm/yyyy'), 'dest3', 2000);
INSERT INTO VACATIONS_TEST VALUES (34, 4, TO_DATE('10/07/2020','dd/mm/yyyy'), 'dest1', 3000);
INSERT INTO VACATIONS_TEST VALUES (35, 6, TO_DATE('12/07/2020','dd/mm/yyyy'), 'dest2', 4000);
INSERT INTO VACATIONS_TEST VALUES (36, 5, TO_DATE('15/07/2020','dd/mm/yyyy'), 'dest3', 3000);
INSERT INTO VACATIONS_TEST VALUES (37, 4, TO_DATE('22/07/2020','dd/mm/yyyy'), 'dest4', 2000);
INSERT INTO VACATIONS_TEST VALUES (38, 2, TO_DATE('24/07/2020','dd/mm/yyyy'), 'dest1', 4000);
INSERT INTO VACATIONS_TEST VALUES (39, 1, TO_DATE('27/07/2020','dd/mm/yyyy'), 'dest5', 2000);
INSERT INTO VACATIONS_TEST VALUES (40, 5, TO_DATE('29/07/2020','dd/mm/yyyy'), 'dest4', 4000);
Create table d1_month (month_contract number(2) primary key);
Insert into d1_month select distinct extract(month from data_contract) from vacations_test;
Create table d2_destination(destination varchar2(40) Primary key);
insert into d2_destination select distinct destination from vacations_test;
Create table fact (month_contract number(2) references d1_month(month_contract),destination varchar2(40) references d2_destination(destination),
nr_vacations number(10),total_price number(20),Primary key(nr_vacations,total_price));
Insert into fact
select extract(month from VACATIONS_TEST.date_contract), VACATIONS_TEST.destination,count(VACATIONS_TEST.IDclient),
sum(VACATIONS_TEST.price)
from vacations_test, clients_test
WHERE VACATIONS_TEST.IDclient=CLIENTS_TEST.IDclient
group by VACATIONS_TEST.destination, extract(month from VACATIONS_TEST.date_contract); // error

Answer 1

It is not that GROUP BY fails, but the fact that primary key is being violated:
SQL> insert into fact (month_contract, destination, nr_vacations, total_price)
2 select
3 extract(month from vacations_test.date_contract),
4 vacations_test.destination,
5 count(vacations_test.idclient),
6 sum(vacations_test.price)
7 from vacations_test, clients_test
8 where vacations_test.idclient = clients_test.idclient
9 group by vacations_test.destination,
10 extract(month from vacations_test.date_contract)
11 ;
insert into fact (month_contract, destination, nr_vacations, total_price)
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.SYS_C007160) violated
SQL>
What to do?
if fact table's primary key isn't correctly set, change it
if primary key is OK but data violate it, then make sure that query returns unique combination of the two primary key columns: [nr_vacations, total_price]
how? Maybe by including the where clause to the query, or ... who knows? I don't, but you should.