I just upgraded to LR 6 standalone thinking I would receive support for Nikon d500 raw files. No luck.
The standalone version is 6.0. What else do I need to buy for this to work?
More than a little frustrated!
If you have just installed Lightroom 6 then you probably have Lightroom 6 .0. Your camera requires at least Lightroom 6 .5. Click on Help->updates to see if you can get the latest update that way. If not, the current date available is Lightroom 6.10 (that's 6.ten). It's available from the following website:
Click on Adobe Photoshop Lightroom 6 and you will see all of the updates listed. You only need the topmost update which is the most recent. That update will take care of both Lightroom CC and Lightroom 6.
What else do I need to buy for this to work?
Nothing more to buy. As Jim said, you just need to install the free update to 6.10.