I am trying to erase my Mac for recycling. It's running macOS High Sierra v10.13.6. None of the suggested methods that I find online work. Help!

See the following Apple Support article for information on erasing a Mac. Skip step 2 as it only applies to newer models and use the other steps instead: What to do before you sell, give away, or trade in your Mac - Apple Support

If the "Use Disk Utility to erase your Mac" section of Use Disk Utility to erase an Intel-based Mac - Apple Support isn't working, you may either want to reply with more details about what is happening when your turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R. 

With more details, someone here may be able to help.

Or Contact - Official Apple Support