How to fix Ora-01427 single-row subquery returns more than one row in Update statement?

As the error says the subquery used to set the updated column produced more than one row.

The most probably explanation is that the UIC column is not a primary/unique key in the table CIELA_STATUS_STG5

Quick check

select UIC, count(*)
from CIELA_STATUS_STG5
group by UIC
having count(*) > 1

You'll see the duplicated keys and you must decide how to recover. E.g. delete duplicates, or use only MAX value in the update or use the newes/oldest rows etc.

Note - to more focused selection of the duplicates, as you do not use the whole table CIELA_STATUS_STG5 in the update use this query, which makes the same restriction as used in the update:

SELECT 
  STG5.UIC, count(*) cnt
FROM CIELA_STATUS_STG5 STG5
WHERE  STG5.UIC IN (
SELECT  CIELA_STATUS.UIC
FROM CIELA_STATUS 
WHERE EXISTS
      (SELECT STG5.OSIGURENI
                         FROM CIELA_STATUS_STG5 STG5
                         WHERE  CIELA_STATUS.UIC = STG5.UIC
                         AND nvL(CIELA_STATUS.OSIGURENI,9999) <> STG5.OSIGURENI)
)
GROUP BY STG5.UIC
HAVING count(*) > 1;

The problem is this part of your query:

SELECT STG5.OSIGURENI
FROM CIELA_STATUS_STG5 STG5
WHERE CIELA_STATUS.UIC = STG5.UIC

Is returning multiple rows. You need a way to pick exactly one row from the many for the insert.

If it gives an acceptable result, the easiest way is to use MIN() or MAX():

SELECT MAX(STG5.OSIGURENI)
FROM CIELA_STATUS_STG5 STG5
WHERE CIELA_STATUS.UIC = STG5.UIC

So your whole query would then be:

UPDATE CIELA_STATUS 
SET CIELA_STATUS.OSIGURENI = (
  SELECT MAX(STG5.OSIGURENI)
  FROM CIELA_STATUS_STG5 STG5
  WHERE CIELA_STATUS.UIC = STG5.UIC)
WHERE EXISTS (
  SELECT STG5.OSIGURENI
  FROM CIELA_STATUS_STG5 STG5
  WHERE CIELA_STATUS.UIC = STG5.UIC
  AND nvL(CIELA_STATUS.OSIGURENI, 9999) <> STG5.OSIGURENI)

If MAX() doesn’t give the proper result, you’ll have to engineer a query that selects the appropriate row from the many that match.