Does that customerID exist in the customer table? Also, it helps to explicitly name constraints. You won't have to guess (or dig out of the data dictionary) what "sys.sys_c001956" is referring to. Are you using one of the more than 400,000 hits Google hits for "Oracle constraints?" Name of a book? Links to Oracle documentation?
Since login is child table which references the master table (Customer) .You have to insert the corresponding record in the master table (customer table) first and then in the child table(login table)
Since you have info about the child and master tables already ,you need not check data dic .
In situations where this constraint error occurs and you don't have a clue as which is the master table then you can use the query
select m.table_name master,c.table_name child
from user_constraints m,
In your situation c.constraint_name='SYS_C001956';