Are you trying to save while moving? There is a bug I forgot to fix where it shows the wrong "Can't save" text while moving (I literally flipped two things around and forgot to change it) so it will claim you're in Hard Mode.
The actual text that's meant to show up is "Can't save your progress while moving"
I tested out, and I can save and load with the "Spooky Seasons on Mata Nui" Cosmetic no problem. Did you play with Takua Adventure Mode on and/or in Freeplay/Endless? Those modes don't allow saving.
If you were playing in Normal Mode with only the "Spooky Seasons on Mata Nui" Cosmetic, I will look into this bug, as that's not supposed to happen at all (It's not even coded to happen).
