Try to use the full path name with tpunmount like:
tpunmount C:\Program Files\VERITAS\Netbackup\db\media\tpreq\<filename>
You can also check bptm logs at (<installdir>/netbackup/logs/bptm) if the log dirs were created. If they were not, I would create at least bptm log dir. It is usefull for...