If after upgrading to a newer version of macOS, the Boot Camp Assistant will not allow the installation of Windows 10, then you should still be able to make the Windows 10 installation flash drive. If you have problems getting High Sierra, then try for Sierra.

The instructions for downloading the upgrade software application is given here. High Sierra is the last version of macOS that does not require APFS, so you should probably at least upgrade to this version. However, this requires using APFS, which will probably have poor performance when installed to the internal HDD.

The newest version of macOS that Apple states is compatible with your Mac is Catalina. You may still be able to license by using the product key from a previous version of Windows. To keep using Windows 10 after installing, you will need to a license, which may require a purchase. What I mean by Windows is the latest version of Windows 10 available, which can be downloaded as an ISO file from this Microsoft website. Having a hard time going back to OS X from Windows 10 on your mac? Here is how to return to OS X from boot camp.Upgrade to a newer version of OS X or macOS before trying to use the Boot Camp Assistant to install Windows to your Mac. You’re essentially carving out an area of your hard drive. Follow the prompts in Boot Camp Assistant to create a partition for Windows. Run Boot Camp Assistant (in the Utilities folder inside the Applications folder) to make sure that you have the latest firmware on your computer. From here, you can choose which OS to load.

To get back to Windows 10, and indeed another way to switch from Windows 10 to macOS, is to restart your Mac and hold down the Option key on your keyboard until a boot menu appears. Your Mac will now restart, and you'll be brought right back to macOS.

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