The error in your original SQL code stems from incorrect syntax and usage of the `CASE` statement along with the `IS NULL` function. The `WHERE` clause is not valid in a `CASE` statement. Instead, conditions inside a `CASE` statement should be specified using the `WHEN` clause.
Here’s the corrected code that should work in SAP Datasphere:
```sql
CASE
WHEN MNTH_END_STK_VAL_STK IS NULL
AND CONCAT(MNTH_END_STK_FSCL_YR,