I was having a similar issue but managed to fix it by downloading the Runescape client and then uninstalling it, and then installing it from the Jagex launcher. When downloading the Runescape Client it said I needed to install something to do with Microsoft DirectX or something like that. I also needed to update my graphics driver. Not sure if any of them were the problem, but it now has let me install it from the Jagex launcher.
This is my first time posting anything so i hope it helps.
15-Aug-2023 12:41:33