DFS823I UNABLE TO OPEN DDN 'IMSACBx' IN (CTL|DLS) ADDRESS SPACE.
Explanation
A permanent I/O error might have occurred during
OPEN. x=A or B depending upon whether IMSACBA or
IMSACBB is active. DLS is included in this message
if the error occurred in the DL/I subordinate address space.
System action
IMS terminates with abend 0071 or 0821.
Programmer response
Ensure that ddname IMSACBx is allocated properly and that the data set specified at IMS system definition time exists. x=A or B.
Problem determination
Check the JCL to ensure proper allocation per the IMS system definition information.