Does this help you answer your question? You also cannot add hashes while ingest is running.
Have you updated to the latest version of Autopsy, and, if so did that fix your problem?
Also, to be clear, we also generically refer to the file its nomenclature, not the exact name, because each time there is an updated version of the NSRL file(s) available, we did not want to have to rewrite the course content. In the example above that John posted, the file you want is “NSRLFile-266m-computer.txt-md5.idx”