If the CA IDMS CV startup JCL does not already specify REGION=0M, then change it so that it does – this will cause the CV to get as much 31-bit storage as it can.
If it is already using REGION=0M, then you need to reduce the size of any of the other database buffers, either VSAM or non-VSAM in order to free up some storage.
Note that VSAM database buffers cannot be adjusted dynamically – they must be changed permanently in the DMCL - followed by refreshing the DMCL or cycling the CV.
Non-VSAM database buffers can be adjusted dynamically with DCMT commands. Remember to make any dynamic changes permanent by making the same change in the DMCL as well.