unable to open raw file from Sony A7R3 with Elements 15

You might also want to try a workaround that I discuss in this post.  I haven't tried it with Elements 15 but it will probably work.

The Sony A7R III requires Camera Raw 10.1 (or later versions).

https://helpx.adobe.com/au/camera-raw/kb/camera-raw-plug-supported-cameras.html

Version 10.1 is only supported by Photoshop Elements 2018 and later so you'll never be able to open Raw files from that camera using Elements 15. Elements 15 only goes up to Camera Raw 9.10 (nine point ten).

https://helpx.adobe.com/au/camera-raw/kb/camera-raw-compatible-applications.html

You can, however, download the free Adobe DNG converter and convert all your A7R III Raw files to DNG format. You can then open the converted DNG files (not the Raw files) in Elements 15. An extra step for you but no extra cost.

https://helpx.adobe.com/au/photoshop/digital-negative.html