Добавил контроллер Supplier

Store/Controllers/SupplierController.cs

+using Microsoft.AspNetCore.Mvc;
+using Store.Models;
+
+namespace Store.Controllers
+{
+    public class SupplierController : Controller
+    {
+        private ProductContext _context;
+
+        public SupplierController(ProductContext context)
+        {
+            _context = context;
+        }
+
+        [HttpGet]
+        public IActionResult Create()
+        {
+            return View(new Supplier());
+        }
+
+        [HttpPost]
+        public IActionResult Create(Supplier supplier)
+        {
+            _context.Suppliers.Add(supplier);
+            _context.SaveChanges();
+            return View(supplier);
+        }
+
+    }
+}

Store/Startup.cs

@@ -59,7 +59,7 @@ namespace Store
                         name: "default", template: "{controller=Product}/{action=Index}");
 
                     routes.MapRoute(
-                        name: "details", template: "{controller=Product}/{action=Details}/{id}");
+                        name: "details", template: "{controller=Product}/{action=Details}");
                 });
         }
     }

Store/Views/Product/Index.cshtml

@@ -27,10 +27,8 @@
                 <td>@product.Category</td>
                 <td>@product.Amount</td>
                 <td>@product.Brand</td>
-                <td><a href="/Product/Details/@product.Id" class="btn btn-sm btn-link">Show</a></td>
+                <td><a href="/Product/Details?id=@product.Id" class="btn btn-sm btn-link">Show</a></td>
             </tr>
         }
     </tbody>
 </table>
-
-

Store/Views/Shared/_MainLayout.cshtml

@@ -27,6 +27,9 @@
                     <li>
                         <a href="/Product/Create">Create</a>
                     </li>
+                    <li>
+                        <a asp-controller="Supplier" asp-action="Create">Create supplier</a>
+                    </li>
                 </ul>
             </div>
         </div>

Store/Views/Supplier/Create.cshtml

+@model Store.Models.Supplier
+
+@{
+    Layout = "_MainLayout";
+}
+
+<h2>Create supplier:</h2>
+
+<hr/>
+
+<form asp-controller="Supplier" asp-action="Create" method="post">
+    <input asp-for="Name"/>
+
+    <input type="submit" value="Save"/>
+</form>

Store/Views/_ViewImports.cshtml

+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers