The effect says that when an effect is activated you can negate it and attach that card. However that doesnt happen, the only thing i can negate is the activation of a spell card.
Eerie Code
The effect in question is this:

Once per turn, during either player's turn, when a Spell Card or effect is activated on the field: You can negate that effect, and if you do, attach that card to this card as Xyz Material.

That effect, according to the original Japanese text, should be read as "When a Spell Card, or the effect of a Spell Card, is activated on the field". Titanic Galaxy only works on Spells, so there's no bug.
So it should work on pendulum scales right?.... But I tried it and it didn't let me activate the effect... My opponent had Qliphort Scout and used the eff to search but i couldn't chain Number 38 to negate it.