‘potions de brassage’ does not mean anything in French. ‘brassage des potions’ can make sense but it really depends on the context.
Both could be correct, with a little stretch of imagination
“Potions de brassage.” would be potions that help brewing things.
Le secret des trappistes ? Ils boivent des potions de brassage avant d’aller travailler !
or (thanks @YohannnV.)
Potion de brassage : Mélanger cette potion à n’importe quel aliment pour en faire un alcool plutôt corsé !
- “Brassage des potions” would rather be “Potion brewing”, that is, the activity of brewing potions.
C’est quoi le cours suivant, Hermione ? Brassage des potions !
That said, brassage is pretty specific to beer, so it would probably be better to find another verb, e.g. “Fabrication des potions”.
Following @YohannnV.’s comment: if your source phrase is indeed “brewing potion” (singular), it could translate as either “potion de brassage” (see above) or “une potion en cours de brassage” (a potion that is currently brewing), but not as “brassage des potions” since in that case potions is generic and therefore can’t be singular.