Hello everyone!
I was trying to install Windows 11 on my computer that has Ubuntu, but I had an error when I tried booting with Yumi: find /menu.lst, /boot/grub/menu.lst, /grub/menu,lst, along with a grub prompt, so I search help and ended up deleting the grub partition and mounting it again.
Then, when I restarted the computer, the screen appeared black with the flashing white "_". If I booted with the Yumi I got this error (before deleting the partition I could ls the partition on this menu): minimal BASH-like editing is supported. Error 17: Cannot mount selected partition, grub>
I can't load any ISO, not even Boot repair disk.
What can I do?
Thank you.

Which release of Ubuntu do you have installed? Prior to your attempt to install windows 11, did you create unallocated space on the drive on which to install windows? Did you select the 'Custom' install option with windows so you could select the unallocated space?

Is the site at the link below to using YUMI what you used? If not, post a link.

https://pendrivelinux.com/yumi-multiboot-usb-creator/

I haven't use YUMI but from what I have seen of it, it is windows software which you can see when you try to download as it has an .exe extension. The UUI software has always been windows as I recall. Do you have some windows version installed along with Ubuntu?

So did you use the YUMI tool to create a bootable installation media for your windows installer and that is when you got the error? I'm not familiar with YUMI, but I would be surprised if it uses a menu.lst file as that hasn't been used with most Linux systems for a decade or more. When you had mounted the USB partition created by YUMI, did you actually have a menu.lst file there?

What 'grub' partition? Did you have a grub partition on your usb? Did you have a grub partition on your Ubuntu install? That would be a bit unusual although a boot partition would be common. If you deleted the partition you won't be able to mount it again because it no longer exists. Was this a boot partition on the usb you were using to create the windows installer for windows? Or was it the grub/boot partition on the hard drive where you had Ubuntu? If that's the case, you will need to reinstall Grub. If you do that, it would be best if you used a usb/dvd of the same version of Ubuntu you have installed.

If you have a usb drive which you can use for the windows installer, you could simply extract or loop mount the windows iso in Ubuntu then copy the contents to the usb drive and put an entry in your grub.cfg file on Ubuntu. There are many sites where you could find a template as an example of a chainload entry. You would need to know if you are installing UEFI or Legacy/CSM. Of course, this won't work now as you seem to have deleted the grub and boot files.