The rar file is still in perfect condition, I just downloaded and checked it right now. Maybe you use an old version of WinRAR or your hard drive is damaged.
i tried it on a different computer and i managed to extract and flash the rom on my phone
But now a different problem arose and that is the OS is bootlooping and it freezes on the umidigi bootup logo
The guide is about flashing in download only mode, without the userdata partition. You should flash your phone in firmware upgrade mode to overwrite the whole software content of your phone.