Apparently we're not able to link it due to items level.
Since the description on item mall is not clear, i bought 3 (2 armors and 1 weapon) Perf. Lap. lvl1.
Got 2 lvl 6 (armor) and 1 lvl 8 (weapon). But they can only be linked to armor lvl 6 and weapon lvl 8 in that case.
I didnt know the 6-10 lvl should be the to-be-linked item lvl, not the lapisia lvl that'd come in the box.
I sent Aeria a ticket asking them to get the Lapisias back and refund me. Hope it's possible.