You did not tell us how TRS-INDX is defined, so there's no way we can answer for sure.
Nope, he/she did give the variable definition (although in cyan....) : 01 TRS-TRANS-MAX-COUNT PIC S9(05) VALUE +10000
Defining a variable you're going to use for subscripting as COMP (or COMP-5 or BINARY) is always a good idea, because it will speed up your code. Defining it as "DISPLAY" implies that the variable has got to be converted to packed first and then to binary (unless IBM introduced a direct conversion from zoned to binary in assembler). However, this is not
the cause of your S0C7.
IF (and only IF)
a) the new variable is defined AFTER the array
b) the code you're performing is moving stuff into the array
then my guess is that you've "blown" past the array boundary and overwritten your TRS-TRANS-MAX-COUNT with something that is not a zoned number.
(Note that WITH TEST AFTER your code may be executed 10001 times....)