11.4 payload don't allow access saves of games at this time. You have to wait to an updated payload to use any homebrew app that access saves.
JKSM will work again when a payload allowing save access will be released.
If you have 11.3, please redownload the payload.
If you have Homebrew Launcher Loader, the app on the Home Menu of the 3DS which launch directly Homebrew please hold down X to download the payload updated for 11.3
And if you have 11.4 and you have CFW (you hacked your 3DS and installed Luma for example), use the CIA version instead.