If it says "in your reserve" then it's a bug as the Amulet is obviously not "in your reserve".
Thanks, that's what I wanted to know. I rechecked the rules and it's clearly stated:
To activate the Mirror of the Seasons, move your sorcerer token
back by “X” spaces on the crystal track. Then transform
“X” identical energy from your reserve (which you discard) into
“X” identical energy of another type from the stockpile.
“X” can be equal to one.
Side question: can "X" be equal to zero? That is, I "activate" the Mirror of the Seasons, but without transforming energies? (For example, to influence the "Heart of Argos" from the expansion)
The online version lets you do it, but not sure whether it's a bug or not.