Quest bugged 'legacy of the vall' cannot complete questlin

I have run the questline 'legacy of the vall' in Act3 (Cruel) but cannot complete because i made mistake which i dropped 'Large soul core' on the ground and it dissapear. so i cant complete this progress because boss never drop this quest item again.

Step.
Legacy of the vall
1. Defeated the boss who dropped 'Larg soul core'
2. The questline show ' Return to infested Barrens and activate the Stone altar'
3. With my mistake, I went to Infested barren but its wrong act3(Not act3 cruel), I confused then dropped the quest item to the ground and it dissapear.
4. I go back to Act3(Cruel), Cannot repeat this questline again , boss never drop "quest item" because the questline stuck here.

Please help check my account. I don't want to play new character.

Same issue boss, let me know if you got yours fixed.