After inserting DVD movie I get message that it would not play in windows media player because of missing DVD decoder. I loaded all drivers off the LG disc, was there not a DVD decoder included or do I have to buy one? LG GH20NS15

This is because Windows has dropped its support to provide Windows Media Player's built-in DVD codec for Windows 10 and above. To a large extent, Microsoft sees that the market is decreasing needs for a DVD drive, not to mention DVD playback service. They take it for granted that not many users may spend extra money for DVD playback since users could easily watch movies from the internet. In addition, Microsoft does not feel like paying a licensing fee to include DVD codec in the operating system. So users may not be able to play DVD via Windows Media Player.

Solution:
Install DVD codec pack. Download the codec pack from the given link: http://www.microsoft.com/windows/windowsmedia/player/plugins.aspx
Step 2: 2: Find a folder on your computer to save the package. And then install it for WMP following the instructions.
Step 3: After installation, insert the DVD and use Windows Media Player to play your DVD disc

Or you can try to download another DVD player program on your PC. You can try Leawo Blu-ray Player, which is a free 4K and region-free player.

Windows Media Player, and many others, will not play everything, regardless of your hardware and it's supporting software.

To overcome this, I recommend a free media player such as VLC (Video Lan) which is free and has a huge amount of codecs and a multitude of other features, including the abilitiy to play most "Region Locked" movies. There are several other good free ones also such as GOM etc, but I prefer VLC due its' mutifaceted capabilities.