after i installed the Xoops, i logged in as a admin and try to modify the website, it will give me this error:

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Forbidden
You don't have permission to access /modules/system/admin.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
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why is that happening? thanks

Double check your root path in mainfile.php.

See this Thread that seems related and see if it helps.