Install RC-20 from the Splice desktop client and restart your machine.
To get RC-20 up and running in Ableton you need to check Ableton's AU/VST settings.
Navigate to :
File and Folder(all Live versions until 10.1)/Plug-ins (Live 10.1 and later)
Make sure "Use AudioUnits/VST" are all checked and hit Rescan.
Windows users will only have the option "VST Plugin Custom Folder", manually navigate to the location you installed your plugin. By default the Windows installation path is:
If you aren't seeing RC-20 in Ableton, try removing all RC-20 Retro Color.dll files on your computer and then start over with the installation via the Splice desktop app.
Make sure that the VST2 custom folder is scanning the 64-bit plug-in file for RC-20. Also make sure that Live is not scanning the 32-bit plug-in, or an old 64-bit RC-20 plug-in at the same time. You need to be absolutely sure that Ableton Live can only find one single file with the name 'RC-20 Retro Color.dll' and that needs to be the 64-bit version, and it needs to be in the VST2 Custom path.