I used parts to make gear from sets, but why did they disappear? Why can’t I equip them?

On the upper-left corner of a gear icon, you can see its “Gear Lv.” That piece of gear cannot be equipped by a character that has a “Character Lv” of a lower value.
Therefore, when selecting a piece of gear to equip from the Character Growth screen, you will not be able to find any piece of gear with a higher Gear Lv, but you will be able to find them in your Item Box.

For example, a piece of gear from the Naamah Set is Gear Lv 180, but it cannot be equipped by Lv 160 character. Therefore, after selecting a piece of gear from the Character Growth screen, then tapping the “Switch” button, the Naamah Set gear will not be displayed on the following Select Gear screen.
For a character to equip a piece of gear, their Character Lv must not be lower than the Gear Lv.

*Level Link can be used to raise a Character Lv to equip gear.
*The Character Lv needs to be equal to or higher than the Gear Lv, regardless of the rarity of the character or gear.
*LR gear can only be equipped and awakened by characters with a rarity of LR+5 or higher. Even if a piece of gear is awakened by another character, it cannot be equipped by a character with a rarity of LR+4 or lower.