Hello! As stated in the Enlightened ability page, if a special factor is not used, the Pokemon will instead evolve at level 30. For Eevee, the Master of Evolution can induce evolution into whatever you'd prefer.
If you'd like an Eevee to evolve sooner, you'll still need the special factor involved in the evolution, either finding a stone, increasing Eevee's affection (in general, affection will increase to an appropriate level after one month), or bringing Eevee to the Moss Rock or Icy Rock.
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If you'd like an Eevee to evolve sooner, you'll still need the special factor involved in the evolution, either finding a stone, increasing Eevee's affection (in general, affection will increase to an appropriate level after one month), or bringing Eevee to the Moss Rock or Icy Rock.