Hey guys,
I got the exact same problem, but Octopi didn’t work too.
I tried to refresh mirrorlist, refresh keyring, clear cache (from octopi), reboot, etc…
What worked is that I followed the instructions from https://aur.chaotic.cx/, so i manually added the key and updated the mirrorlist (even tho the “refresh mirrorlist” in Garuda assistant should have done that ?)
That worked, I hope it helps 
Cheers !