Assembly 'abc' was not found

Assembly 'abc' was not found. Error on line
xmlns:abc="clr-namespace:abc;

    <Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xml:lang="en-US"
        xmlns:abc="clr-namespace:abc"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2006"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        xmlns:local="clr-namespace:abc"
        x:Class="abc.Window1"
        x:Name="Window"
        Title="smart planner" >
</window>