Obviously, 'anvertraubar' does exist. Dictionaries just don't list all possible or rare derivations of words. But 'anvertraubar' is a correctly formed and comprehensible adjective. It requires a copular verb (Kopulaverb: sein, werden, bleiben,…) to which it is a predicative adjective (anvertraubar sein), and this will usually require a dative object to form a complete statement:
{Etwas/Jemand}ᴺᴼᴹ ist {jemandem/etwas}ᴰᴬᵀ anvertraubar.
Das Geld ist dir anvertraubar. (not: 'bei dir anvertraubar')
Attributive use in a noun phrase:
der/die/das {jemandem/etwas}ᴰᴬᵀ anvertraubare {etwas/jemand}
Technically, the adjective describes the subject or the noun, but on the semantic level it is of course a statement about the person/thing that can be entrusted with the subject. That's just in the meaning of the word: The prefix 'an-' in 'anvertrauen' signals the transfer of something in a trustful manner.
But there are also examples of different uses, e.g., to describe that trust can be put into something that is desirable or expected to happen in something. It is here somewhat close to "zutraubar" (which is also not listed in your dictionary).
"Normalerweise ist es dem Prozess der Vertrauensbildung und der Pflege
der Beziehung gut anvertraubar, dass sich die in neuen Bekanntschaften
anfangs oft geschärfte Wirkungssuche schließlich verläuft." (Nicole Diercks: "Nie mehr weg von mir! Stärker nach Burnout", 2015)