No boot device found after changing boot mode from Legacy to UEFI in DELL 5559

@Old FarmerAs I said in my earlier post, when dealing with a disk that contains an OS, a simple conversion from MBR to GPT isn't enough to make your OS disk bootable in UEFI mode.  As for creating a path to Bootx64.efi, you can't just type that and expect it to work.  The Bootx64.efi file you're pointing to has to actually exist on your disk, specifically residing in an actual ESP (EFI System Partition).  That is created by default when you install Windows in UEFI mode in the first place, and it also would have been created if you had used the MBR2GPT utility I mentioned above, which takes additional steps to convert an OS disk from supporting Legacy BIOS booting to UEFI booting.  But it would NOT have been created on a disk originally set up for Legacy BIOS booting, nor would a simple GPT conversion have created it.  And since Legacy BIOS mode doesn't support booting from GPT disks and you can't convert a GPT disk back to MBR, you're basically stuck.  In theory you might be able to manually create an ESP on your GPT disk and get the necessary bootloader files copied onto it, but I'm not sure that would work, and it would almost certainly be impractical to try to help you to do that over an Internet forum.  For one thing, Microsoft's guidelines specify that an ESP has to be located before the OS partition, not after it, which is harder to achieve when you have existing partitions on disk.

At this point I would probably recommend that you back up whatever data you need on your disk, then switch your system to UEFI mode, boot Windows 10 installation media, and at the step in the wizard asking you where to install Windows, delete ALL partitions on your internal disk until it shows as just a single "Unallocated space" entry, then choose to install there.  Windows will set itself up properly for UEFI booting upfront in that case, including creating the appropriate partitions.  If you know what you're doing, you could first capture an image backup of your Windows partition before performing a clean installation, so that AFTER you complete a clean installation, you can restore just the Windows partition from that image backup over the new one that was created during the clean installation, WITHOUT touching any of the other partitions created on the disk.  In that case, your system would maintain UEFI boot support, because your disk will now have an ESP that you would NOT be overwriting during the image restore, but you'll have your Windows environment back rather than having to rebuild everything.  But if you DON'T know how to do that, then that too is something that's probably more than I really want to try helping someone achieve by posting back and forth on a public forum.  I just wanted to suggest that in case you would know how to take that idea and run with it.  Macrium Reflect is a popular imaging tool and it has a free version that would be perfectly usable for this purpose, fyi.