grub cannot detect detect the live usb. how to do i access it?
AFAIK, it never will, CMIIW tho
if you want to boot from live usb, shouldnt you use “boot selection” menu from your bios? or change the boot order from bios ? 
i zm dual booting, i can only rearrange the list in my bios , can’t access from there
assuming, you already created bootable live usb (via whatever tools you use)
can you just make the usb as first boot in bios? (you can always change back later)
on my end, i mostly use that way, so instead booting grub first, it always boot the live usb first
i put it first and then it wont be there the next time, it will be at the bottom
seems like bios mechanism to me, since it’s usb, if you’re unplug them, bios just detected it as invalid device, and remove it from entries, and when you plug it again, it just start at bottom one
some bios have this AFAIK
so i think the only way is for (consistent) you need to find a way to go to boot selection when booting
most key to enter this was F10 or F11, but you might research yourself for your motherboard, after that just pick the usb, then it should be boot your live usb, if it’s not detected by anychance (on the list) you might consider other way to create the bootable usb
I am using ventoy
It’s the BIOS that lets you boot from a USB drive. On my MSI PC, you have to press F11 to display the boot sequence and choose what to boot from.
I also use Ventoy without any problems.
I’m going to assume you have saved the USB flash drive as primary boot device in the BIOS? If so this might sound weird but try a different USB port. I have one laptop an HP that will only boot from a USB flash drive with an OS (typically Linux) from one specific USB port.
It could also be an issue of timing (yes that really can be a thing). Try inserting the USB at different juncture in the start up sequence. This may take some trial and error.
see this