Cannot Insert into SQL Database in Oracle SQL Developer

I have created a table using following code:

CREATE TABLE GLASACKO_MESTO(
	"ID_glasackog_mesta" NUMBER(10, 0) PRIMARY KEY,
	"Broj_registrovanih_biraca" NUMBER NOT NULL,
        "Broj_glasackog_mesta" NUMBER NOT NULL,
        "Izborna_jedinica" NUMBER NOT NULL
);

CREATE INDEX GLASACKO_MESTO_BROJ_REG_B ON GLASACKO_MESTO("Broj_registrovanih_biraca");

and also:

CREATE SEQUENCE "GLASACKO_MESTO_ID_SEQ"
	MINVALUE 1
	MAXVALUE 9999999999
	INCREMENT BY 1
	START WITH 101
	CACHE 5
	NOORDER
	NOCYCLE;

CREATE OR REPLACE TRIGGER "GLASACKO_MESTO_AUTO_PK"
    BEFORE INSERT
    ON GLASACKO_MESTO
    FOR EACH ROW
BEGIN
    :NEW."ID_glasackog_mesta" := GLASACKO_MESTO_ID_SEQ.NEXTVAL;
END;

As i can see, it looks like everything was created successfully, but when try to execute following INSERT command, i get error.

INSERT INTO GLASACKO_MESTO(Broj_registrovanih_biraca,Broj_glasackog_mesta,Izborna_jedinica)
values ('2','3','4');

And also tried setting manualy ID

INSERT INTO GLASACKO_MESTO(ID_glasackog_mesta,Broj_registrovanih_biraca,Broj_glasackog_mesta,Izborna_jedinica)
values (1,'2','3','4');

But it didn't make any difference. How can I fix this?

You used string literals '2','3','4', but the column types are defined as number. You probably want this:

INSERT INTO GLASACKO_MESTO(Broj_registrovanih_biraca,Broj_glasackog_mesta,Izborna_jedinica)
values (2,3,4)

Additionally, the error message says IZBORNA_JEDINICA is an invalid identifier. That means there's likely a typo or odd unicode character either in the SQL or the table definition so that this column name doesn't match what was defined on the table.

You created the table with case sensitive columns (since you put quotes around the column names), so you always need to reference the columns with quotes and the same case. Recreate the table without the quotes around the column names. It will save you time and headaches.

Check this example:

create table TABLE1 ("column1" VARCHAR2(100));
Table TABLE1 created.
INSERT INTO TABLE1(COLUMN1) VALUES ('koen');
ORA-00904: "COLUMN1": invalid identifier
INSERT INTO TABLE1("column1") VALUES ('koen');
1 row inserted.

Now create the table with no quotes around the column name:

create table TABLE2 (column1 VARCHAR2(100));
Table TABLE2 created.
INSERT INTO TABLE1(COLUMN1) VALUES ('koen');
1 row inserted.

More info here