its working for me...
windows 10 pro with deferred updates 64-bit
gimp 2.10.8 64-bit
riot 0.6.2 (32-bit as thats all thats offered)
riot plugin installer detected gimp right out of the box
the install tries to install riot to "C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins\riot\" and i have no issues running it.
try running the installer it as admin, that should fix any permission issues for the installation.
my process for installation.
double click the installer (not a admin install)
destination folder is left alone (default value: "C:\Program Files (x86)\Riot\")
check "the gimp"
plugin dir is left default "C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins"
if you want to manually install from the installer
1) use 7zip (or something else that can open the EXE as a archive) to open the installer
2) open the $_23_ folder, drag the riot folder into the gimp plugin folder
3) launch gimp (close it first if already open)
also checking your program files permissions may be a wise idea.
also does your plugin folder look like this?
https://ibb.co/N66dHMr