Hello Nader,
The error message you are getting is telling your clearly what the problem is (concatenated index will exceed 255).
What is odd is the:
MODIFY ("TCTHIGH" VARCHAR2 (003999)
MODIFY ("TCTLOW" VARCHAR2 (003999)
Please run the following SQL (post output here) which will describe all indexes on the table in question. From that you will be able to determine which index(es) is/are impact by the ALTER COLUMN.
Select table_owner, table_name, index_name, column_name, COLUMN_LENGTH
FROM dba_ind_columns
Where table_owner='SAPSR3' AND table_name='/BI0/ASRSA_DS100'
Order by table_owner, table_name, index_name, column_position ;
There is a way forward with this but in such circumstances I would open a message with SAP anyway (to understand why).
Kindest Regards,
Amerjit