your code is correct because in some cases it does not work, I also do not know where the error comes from but this can help you solve the problem, move the code in the head of the view like this:
index.cshtml:
@using expert.Models;
@{
ViewBag.Title = "Create";
Layout = "~/Views/Shared/_Layout.cshtml";
Manager db = new Manager();
ViewBag.FORM_PRESTATION = new SelectList(db.T_BDE_PRESTATION_PRES.OrderBy(p => p.PRES_INTITULE).Where(p => p.T_B_PRES_ID == null), "PRES_ID", "PRES_INTITULE");
}
<div class="form-group">
<label class = "w3-blue" style ="text-shadow:1px 1px 0 #444">Domaine:</label>
<div class="col-md-10">
@Html.DropDownList("FORM_PRESTATION", null, htmlAttributes: new { @class = "w3-select ", @required = "true" })
</div>
</div>