You must apply several IBM APARs before you apply LU15786 and start using the XCOM r12.0 Cloud Services enhancement.
XCOM Data Transport for z/OS
Apply the following IBM APARs before using the XCOM r12.0 Cloud Services enhancement:
IBM APAR OA64874: NEW FUNCTION: GDKUTIL METADATA SUPPORT (part of z/OS 3.1 base)
IBM APAR OA65224: NEW FUNCTION: CDA API ENHANCEMENTS
IBM APAR OA65227: GDKWRITE HWTSLST 301 0C4 STORAGE (part of z/OS 3.1 base)
IBM APAR OA65717: CLOUD DATA ACCESS WRITE WRITELARGEOBJECT CHUNKSIZE PUT
IBM APAR OA65718: CLOUD DATA ACCESS GDKWRITE CUSTOM HTTP HEADER
IBM APAR OA66943: CLOUD DATA ACCESS GDKIGET DATASET ABEND 0C4 STREAM_TO_FILE FCLOSE
IBM APAR OA65451: CLOUD DATA ACCESS GDKUTIL LIST OBJECT SIZE
IBM APAR OA66415: NEW FUNCTION - DFSMSDFP CDA TLS 1.3 SUPPORT