How do I install Xcode if I'm getting this message "Xcode can’t be installed on “Mac HD” because macOS version 10.14.4"?

Recent versions of Xcode require macOS 10.14.4 or higher to run. Since you have 10.13.6, you can't run that version of Xcode. With 10.13.6 the most recent version of Xcode you can use is Xcode 10.1.

Also, Macs made in 2010 cannot run macOS 10.14.4, which is why that version of macOS is not available to you as an update. Your Mac can't be updated past macOS 10.13.x.

You can do one of the following:

• Stick with Xcode 10.1. You can download it from https://download.developer.apple.com/Developer_Tools/Xcode_10.1/Xcode_10.1.xip (you'll need to log in to your Apple developer account).
• Upgrade to a newer Mac that will run macOS 10.14.4. You can find a list of the system requirements at Wikipedia: https://en.wikipedia.org/wiki/MacOS_Mojave

I ran into this issue lately too. I own an early 2011 MBP and was not able to update to Mojave either and therefore I could not install the latest Xcode.

While it isn't supported officially, there's still a way to update to Mojave and run the latest Xcode. dosdude1 has created a patcher that allows you to install Mojave on your (officially) unsupported Mac, you can find it at http://dosdude1.com/mojave/ - they also have a patcher for Catalina

Please note

• This is probably a violation of Apples ToS
• If you want to release your application in the App Store you might run into troubles (unlikely, albeit, still conceivable)
• You'll have to trust dosdude1, the patches are not open source, as far as I can tell
• The machine is not guaranteed to work as it did before