Q:

Error : ORA-00907: missing right parenthesis

I'm trying to create a simple table but it's giving me an error says: 
Error starting at line : 1 in command -
CREATE TABLE dj_abonent
(
nr_klienti int NOT NULL,
emer_klienti varchar2(10),
sasia_cel int
CONSTRAINT dj_klientID PRIMARY KEY(nr_klienti)
)
Error at Command Line : 6 Column : 35
Error report -
SQL Error: ORA-00907: missing right parenthesis
00907.  00000 -  "missing right parenthesis"
*Cause:    
*Action:
Error starting at line : 1 in command -
CREATE TABLE dj_abonent
(
nr_klienti int NOT NULL,
emer_klienti varchar2(10),
sasia_cel int,
CONSTRAINT dj_klientID PRIMARY KEY(nr_klienti)
)
Error at Command Line : 1 Column : 14
Error report -
SQL Error: ORA-00955: name is already used by an existing object
00955.  00000 -  "name is already used by an existing object"
*Cause:    
*Action:

A:

Your have the second error because you use dj_klientID as name for both of the constraints.  They have to be unique. 
Try renaming one of the two constraints and also your second query will be fixed.