IDSTIOST03 - ASSGN card for "SYSnnn" is missing

Reason:

(VSE only) One of two possibilities for error exists:

The command SET SOURCE DDNAME SYSnnn specified a file-name that was present on the IDSYSFT assembly but was not ASSGN'd in your JCL. Specify nnn in your JCL or change the SET statement to correspond with your JCL.
The JCL was misspecified without an ASSGN statement for SYSnnn.

Action:

Correct the errors and re-specify.