Cause:

Silly ! Connection string and password should be placed inside quotes

SQL> create database link dblink_prod_qa connect to test identified by password using orcl19x;
create database link dblink_prod_qa connect to test identified by password using orcl19x
                                                                                 *
ERROR at line 1:
ORA-02010: missing host connect string

Solution:

SQL> create database link dblink_prod_qa connect to test identified by "password" using 'orcl19x';

Database link created.