public function destroy($id)
    {
        $user = User::find($id);
        $user->delete();

        return redirect('/admin')->with('success', 'User has been deleted');
    }

Route::post('/admin/delete/{id}', 'AdminController@destroy')
    ->middleware('is_admin')
    ->name('admin.destroy');

<td>
<form href="{{ route('admin.destroy', $user->id)}}" method="post">
   @method('DELETE')
   @csrf
   <input class="btn btn-danger" type="submit" value="Delete" />
</form>

The DELETE method is not supported for this route. Supported methods: GET, HEAD.

Route::delete('/admin/delete/{id}', 'AdminController@destroy')
    ->middleware('is_admin')
    ->name('admin.destroy');