Short answer. Yes. Article 385 of the Civil Code allows a petition for declaration of absence from anyone who has over the property of the absentee some right subordinated to the condition of his death — and an inheritance you only receive once your missing relative is declared dead falls within that category.
What the law says
Those who may have over the property of the absentee some right subordinated to the condition of his death.
Civil Code, Article 385 — Who May Ask for Declaration. Read the full provision →
A right conditioned on death qualifies
Article 385 names, as one of four categories entitled to petition, those who may have over the property of the absentee some right subordinated to the condition of his death. An inheritance that only becomes yours once your missing relative is declared dead is exactly that kind of right — one that is conditioned on the absentee's death, and therefore falls squarely within this fourth category.
Note: this article is about a declaration of absence, not death
It is worth being precise about what Article 385 lets you petition for: a declaration of absence, which is a different legal status from a declaration of presumptive death. The article gives standing to petition for absence to someone in your position, but it does not by itself equate absence with the death that your inheritance is actually conditioned on. The declaration of absence deals with the stage where the person's fate is still unknown; whether and when the law will treat the absentee as dead for succession purposes is a further, separate question, and it is that later question your inheritance actually turns on.
The other three categories, for comparison
The other three categories Article 385 recognizes are the spouse present; the heirs instituted in a will, who may present an authentic copy of it; and the relatives who may succeed by the law of intestacy. Your standing as someone with a right conditioned on the absentee's death is a separate, independent basis from all three of these — you do not need to also qualify as a spouse, instituted heir, or intestate relative to bring the petition.
What this article does not resolve
Article 385 establishes who has standing to ask for the declaration; it does not itself set out the grounds for declaring someone absent, the procedure the petition follows, or what legal consequences flow from a declaration of absence once granted. Those questions are addressed elsewhere, and knowing you have standing under this article is only the first step in what such a petition would involve. Standing, in other words, is all Article 385 hands you: it settles who may ask, not how the case proceeds or what follows.