My 8 GB USB drive has been formatted and is free of all errors. Yet, I am unable to write the ISO on to the drive using Rufus. What's wrong?
First of all, you should really check what the log from Rufus says, as it will give you an error code that you can check. Just click on the last blue button that appears left of Start to access the log.
Also, please be mindful that there exist a comprehensive FAQ for Rufus, that has a specific entry for this error telling you what you should do.
If I were to take a guess, I'd say that you probably enabled the Controlled Folder Access feature from Windows Defender, which of course prevents anything but whitelisted applications (such as the native ones from Microsoft) from partitioning a drive. So you will either have to disable Controlled Folder Access or whitelist Rufus. However, without seeing the error from Rufus' log, it's impossible to say what your issue might be. For instance the FAQ also has a list of applications that, similarly to Controlled Folder Access, may prevent partitioning access to a drive.
So, the answer here really is: check the Rufus FAQ and follow the advice it gives you.
Suggest you try another app, to troubleshoot whether the flash drive is starting to fail or if it is a Rufus issue. https://itsfoss.com/create-live-usb-of-ubuntu-in-windows/ provides step by step instructions to:
A) Download the ISO for Ubuntu 18.04 AND make sure it downloaded AOK with checksum verification
B) Download the Universal Installer app for Windows from Pendrivelinux.com
C) Install the ISO image with FAT32 formatting as shown in the photo from the first link. If the install was successful, then the USB drive is OK and it is a Rufus issue to address by emailing firstname.lastname@example.org after reviewing the Rufus FAQ.