My late 2012 MacBook Pro is running Catalina. I used the Bootcamp Assistant to format a blank flash drive, copy my Windows 10 ISO file (downloaded from the provided Bootcamp assistant link) onto the flash drive, download the latest Windows software support from Apple, and create a new Windows partition to install it on (in other words, I checked all three boxes on the bootcamp assistant menu). It took about 4 hours for the computer to finish it all, but after I confirmed my password a few times and it finally finished, my computer restarted automatically, booting to the windows partition by default. But, instead of any kind of Windows 10 install menu (even though it seems like the install should have already been completed by the bootcamp assistant), it just says "NO BOOTABLE DEVICE - INSERT BOOT DISK AND PRESS ANY KEY."
I restarted and booted back to OS X, removed the Windows partition using the bootcamp assistant, and then did it all again, even re-formatting and re-copying windows to the newly blank flash drive. Same result. Then, I tried it again using a different flash drive, with the same result. When I restart the computer and hold alt/opt, It only shows my OS X drive, as well as "Windows" and "EFI BOOT". When I boot onto "Windows", it takes me to what looks like the install menu for Windows 10 on a blank computer, but when the option comes up for which drive to install Windows on, and I try to click the 300GB partition that I created using Bootcamp, it says "Windows cannot be installed on this disk."
I'm not sure what's wrong with it. A few months ago, when I was running Mojave instead of Catalina, I installed Windows 10 in the exact same way, and it worked fine.