If not knowing the admin password is why you want to uninstall ePO, it can be fixed by logging a case with McAfee Support. They would help you reset the password.
Having said that it may require further investigation as to why the uninstall is failing with the error you mention.
I know we have posted a KB article that discusses how to do a manual uninstall of the server. Please see KB article KB66586. This will walk you thru step by step how to uninstall the ePO server software.