There can be a couple of causes for the error you encountered. The PST may be password protected or corrupted. ScanPST can somtimes repair the PST.
What version of Clearwell are you on? How large is the PST you are trying to process?
Hello it's worth reviewing the logs in the logs directory, check out the PST* prefixed logs - it may give you a bit more specific information, although the pre-processing errors report should also help to identify what is going on.