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xuanyuu, welcome to the XOOPS forum!
It seems something went wrong during uninstall and some tables are left behind in the database.
You need to erase them first, before a new install will succeed.
Use eg phpMyAdin to drop this tables. You can look up the names (without prefix) in the /modules/wfdownloads/sql/mysql.sql file.
Put also PHP debug on in the preferences, so you are informed at errors during the installation.