This mobile game feels cheap and poorly made. It's disgustingly rigged so that opponents have perfect answers to your deck, with perfect sequences and draws.

Meanwhile, you, as the FTP player, can only watch them play without being able to interact.

I advise anyone of decent character not to spend any money on it, especially considering the number of bugs they haven't fixed.

Este juego para móviles se siente de mala calidad, a parte, esta asquerosamente trucado para que los oponentes tengan las respuestas perfectas a tu baraja, con la secuencia y robos perfectos.
Mientras tú, como ftp, ves como juegan sin poder interactuar.

Aconsejo a cualquier persona de bien que no meta dinero y más teniendo en cuenta la de bugs que no son capaces de arreglar.

Multiple possibilities:

You don't have a legal target (nonblack creature required)
You are not allowed to pay life (Yasharn, or Karn's Sylex, etc.)
You are not allowed to cast it on that player's turn (Voice of Victory, or Grand Abolisher, etc.)
You are not allowed to cast that instant (Lavinia, Azorius Renegade, etc.)
You are required to pay an increased cost and cannot pay (Thalia, Guardian of Thraben, etc.)