Hrm thank you for your answer here, I was not correct in my contribution here
I run debain 4 and follow the guide
I dl the ISPConfig-126.96.36.199-rc1.tar.gz and unpack it again, then in the dir
/ispconfig3_install/install I could find the uninstall.php.
That one was I in need to chmod first, then run it with error I wrote.