Cannot convert float to int?

Mathf.Lerp returns a float so it's not wonder you get this error.

 manaValue.value = (int)Mathf.Lerp(manaValue.value, (float)player.playerMana / (float)player.playerMaxMana, 0.5f);

or

    manaValue.value = Mathf.RoundToInt(Mathf.Lerp(manaValue.value, (float)player.playerMana / (float)player.playerMaxMana, 0.5f));

However, I don't know if the lerping would actually change the manaValue when rounding the end result. A better way would be to make the manaValue a float and display it with Mathf.RoundToInt if you don't want decimals in your GUI. Generally speaking, if lerping and similar operations are involved, it's best to make the variable a float and not an int. If you really want to keep it as an int, you can also consider slowing the rate at which it decreases, for example with a CoRoutine that waits a certain amount of time between decreasing the amount by 1.

 IEnumerator DecreaseMana() {
      while(true) {
           manaValue.value --;
           yield return new WaitForSeconds(1f);
      }
 }