Necril is an uncommon language in Pathfinder 2e, meaning that you need some special feature to qualify for it.
Are there any existing ways for a player character to have access to the Necril language?
You could select these if you have the slots available if you are from a region where it is common, so in this case if you had an Underdark origin, you could possibly select Necril. Or, if your DM allows, you could have a local expert in the language teach it to you.
From the core rulebook:
You can choose these languages from the list presented in your character’s ancestry entry and from those available from your region or ethnicity.
Languages that are uncommon (see Table 2–2 and Regional Languages) are most frequently spoken by native speakers, but they are also spoken by certain scholars and others interested in the associated cultures.
You can also select the Multilingual feat and directly select uncommon languages:
It is possible for your character to learn languages later in their adventuring career. Selecting the Multilingual feat, for example, grants a character two new languages chosen from those listed in Table 2–1: Common Languages and Table 2–2: Uncommon Languages.
The Emissary background grants the multilingual feat:
As a diplomat or messenger, you traveled to lands far and wide. Communicating with new people and forming alliances were your stock and trade. Choose two ability boosts. One must be to Intelligence or Charisma, and one is a free ability boost. You’re trained in the Society skill and a Lore skill related to one city you’ve visited often. You gain the Multilingual skill feat.
Multilingual feat for reference:
You easily pick up new languages. You learn two new languages, chosen from common languages, uncommon languages, and any others you have access to.
There are also some racial archetypes that may grant access, depending on the DM ruling on availability to a scholar or other source for the Necril, as it doesn't explicitly grant it, but states "from those available to you".
Your ancestors have traveled from place to place for generations, never content to settle down. You gain two additional languages of your choice, chosen from among the common and uncommon languages available to you, and every time you take the Multilingual feat, you gain another new language.
Characters from these ethnicities speak Common if they come from the Inner Sea region, and some ethnicities give access to a regional language or another uncommon language (page 65). Half-elves and half-orcs arise across all human ethnicities and bear the features of both those ethnicities and their non-human parents.
It also does not state which specific ethnicities might have access to Necril.