XAML is not loading, d:Height & d:Width are not recognized?

<UserControl x:Class="PressureVessels.UserControls.NozzleUC"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:sys="clr-namespace:System;assembly=mscorlib"
              xmlns:UC="clr-namespace:PressureVessels.UserControls"
             xmlns:vm="clr-namespace:PressureVessels.ViewModel"
             xmlns:l="clr-namespace:PressureVessels"
             xmlns:c="clr-namespace:PressureVessels.Controls"
             mc:Ignorable="d" d:Height="1267" d:Width="698"><UserControl.DataContext><vm:NozzleVM/></UserControl.DataContext>

It used to work, but all of a sudden it has stop working. It doesn't recognize blend.