Did Not Receive Tome of the Eternal After Completing Eternity's end Chapter 5 The Crown of Wills

A character will only receive Tome of the Eternal as a reward from the quest The Crown of Wills if they have one of the four crafting professions that can craft Legendary base items: Blacksmithing, Jewelcrafting, Tailoring or Leatherworking.
The character must also have a high enough skill level to craft the Legendary base items, for example Shadowlands Blacksmithing (100).

A character without one of the above four crafting professions, or who does not have a high enough skill level in that profession will not receive the Tome of the Eternal as a reward from the quest.
Levelling up their profession to the appropriate skill level will allow them to then acquire the Tome of the Eternal later from Vilo <Enlightened Quartermaster>.