Data annotation validation attribute for DropDownList with default value in ASP.Net Core MVC

I am making a form which I validate with EF Data annotation but for the data "Unit of Measure SAT", it does not put the legend that I indicated in the data annotation, which should be: "The Unit of Measure is required" and I presents "The value is invalid." And for the other controls if the legend presents well

Any idea why in this field the one I indicate does not present

CLASS
[Display(Name = "Unidad de Medida SAT")]
[Required(ErrorMessage = "La Unidad de Medida es requerida")]
public int IdSATUnidadMedida { get; set; }

CONTROL
<div class="form-group">
    <font color="red" size=5>*</font> <label asp-for="IdSATUnidadMedida" class="control-label"></label>
    @Html.DropDownListFor(u => u.IdSATUnidadMedida, unidades, "-- Seleccione Unidad de Medida --", new { @class = "form-control" })
    <span asp-validation-for="IdSATUnidadMedida" class="text-danger"></span>
</div>