I'm new to ASP.NET MVC. I'm trying to get the ITEMNAME from tbl_inventorycontrol.

@model RSMI.Models.Database.tbl_STOCKCARD


@Html.LabelFor(model => model.ITEMCODE, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
    @Html.EditorFor(model => model.ITEMCODE, new { htmlAttributes = new { @class = "form-control" } })
    @Html.ValidationMessageFor(model => model.ITEMCODE, "", new { @class = "text-danger" })
    <br />
    <div class="form-group">
        <div class="col-md-offset-24 col-md-12">
            <input type="button" value="Search Item"  onclick="location.href='@Url.Action("SearchItemCode","Stockcard")'" class="btn btn-primary btn-block" />

It should it should get the ITEMCODE value from tbl_stockcard. Then in stockcard controller, when I click search button, it should get the itemname in tbl_inventorycoller equal to the tbl_stockcard.ITEMCODE.

It's not entirely clear what you want to do with the filter result. Seems you mismatched you return statements. You want returning View with your item if found. Please consider this approach

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Searchitem(tbl_inventorycontrol tbl_Inventorycontrol )
{
    if (ModelState.IsValid)
    {   
        var item = db.tbl_inventorycontrol.Where(tbl_STOCKCARD => tbl_STOCKCARD.ITEMCODE == tbl_Inventorycontrol.SUPPLIES).FirstOrDefault();
        
        if (item != null)
        {
            return View(item);
        }    
    }
    return View(tbl_Inventorycontrol);
}