ORA-00904 & EXP-00091

Solution
To implement the solution, please execute the following steps:

1. Set NLS_LANG on OS, then exp again:
$export NLS_LANG=american_america.ZHS16GBK
$echo $NLS_LANG
$exp system/passwd full=y

2. You can add statistics=none:
$exp system/passwd full=y statistics=none