DFAX compilation error: 'Unit not open'

edit

When creating a dfax using the PSSE API, the following output is written to console and a terminal is opened requesting input.

Processing the Subsystem Description Data File... Out of file data--switch to terminal input mode

The terminal does not go away regardless of input until I type in 'END', after which the process aborts and the following error message is written to the console:

Unit not open. C:\Users\username\Documents\filename.mon (OpnApplFil/OPNSEA)

What could be causing this error message?