Ubuntu startup disc is not working

I downloaded the .iso file for Ubuntu 20.04, to create a startup disc. I followed the instructions on the main Ubuntu site. The first problem was that Startup Disc Creator would not allow the named file to be accepted. The file name ended .iso.part. I had to change the name of the downloaded file to .iso for it to be accepted. My USB drive stick then was loaded with the file. When I powered up my laptop with the USB stick in place, and then selected the USB drive as the One Time boot option, the laptop display of Grub items showed options. When I selected any of them I got an error message saying that the kernel had not been loaded. What can I have done wrong?

"The file name ended .iso.part"

.part is a convention used by web browsers to indicate that it's not the complete file, and/or that the download is still in progress.

In other words, your description suggests that you failed to download the entire .iso file.

Try downloading it again. It's a big file -- wait for the download to complete, and for the correct filename to appear in your File Manager.