CA IDMS CONFIG member JOB17 copies SYSTEM 99 elements to SYSTEM 90 but there is no SYSTEM 90 defined.
Release : 19.0
Component : CA IDMS/DB
SYSTEM 90 is a test system created in STEP17 during a base install.
When doing an upgrade, STEP17 just updates the existing system 90 with the latest task and program definitions.
If you want to create SYSTEM90 generate JCL for a base install and run CONFIG job STEP17.
SYSTEM 90 is not needed but existing systems should also be updated with the latest tasks and programs using the input from STEP17.
See the documentation on STEP17