I have received a same error "Space not specified for allocation of dataset", but in DD statement it has mentioned DISP=MOD. the dataset is already created and 80% of the space has been used. The data has to be append in the dataset.
First, you should NOT re-open old threads with a new question. This thread was last used 8 years ago! You should open a new thread.
As Steve said, if the dataset isn't cataloged you would probably need a space parameter. You would also require VOL=SER=?????? so the system knows where to place the dataset. If the dataset already exists and is not catalogued, then the VOLSER in JCL must reference the disk where it exists - otherwise it will be considered to be a new dataset.