In Umbra forest at the Forgotten Tomb, there was this seal that had to be opened. I could choose me, Jinxie, my party pets or do nothing and I tried all of them but it still didn't work. Is it meant to be like that or is there some sort of bug?

it's meant to be a pet who has a high stat (200+) in what the seal wants- each of the four seals wants one of the four main stats, depending on which seal it is (of Alacrity, Perception, Courage, and Cunning). There's a thread here if you just want the answer (which stat goes to which word is on page 2, in post #15)