Used Bootcamp to get Windows 10 on MacBook Pro (Mid-2012, Catalina) says "no bootable device - insert boot disk and press any key"
When installing Windows 10 on your model Mac, you should verify the Windows 10 USB installer sees Disk 0 as using the GPT scheme. When the first Windows 10 installer screen appears, press the key combination shift+F10. This will open a Command Prompt Window. Enter the command given below.
echo list disk | diskpart

Disk 0 row should show a * character in the Gpt column, as shown below.

To close the Command Prompt window, enter the command exit.
Also, when installing Windows 10 on your model Mac, you should EFI boot the Windows 10 USB installer. If you are using the Mac Startup Manager, then choose the EFI Boot label under an external drive icon. If you incorrectly BIOS boot from the Windows 10 USB installer, then you should get the error message Windows can't be installed on drive 0 partition N. (Show details), when choosing the partition where you want Windows installed. If you select (Show details), then you should get the popup window shown below.

To summarize, you need to EFI boot from the USB flash drive and Disk 0 needs to be using the GPT scheme. In other words, Disk 0 can not be hybrid partitioned.