You have attempted to use the SIZE OF expression as part of a receiving item.
In this COBOL system, you can only use the SIZE OF expression as a sending item. For example:
01 firstName string
01 totalLen pic 9(3).
...
add 10 to SIZE OF firstName. *> this is incorrect
add SIZE OF firstName to totalLen. *> this is correct
Note: You can only use the SIZE OF expression with single-dimensional managed arrays or strings.