Test_Kvalif/WpfApp1/Windows/AdminWindow.xaml

<Window x:Class="WpfApp1.Windows.AdminWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1.Windows"
        mc:Ignorable="d"
        Title="Администратор" Height="450" Width="800" WindowStartupLocation="CenterScreen" Loaded="WindowLoaded">
    <Grid>
        <TabControl>
            <TabItem Header="Сотрудники">
                <Canvas>
                    <DataGrid Name="UserTable" Width="Auto" Height="371" Canvas.Left="10" Canvas.Top="10" IsReadOnly="True" GridLinesVisibility="All" AutoGenerateColumns="False">
                        <DataGrid.Columns>
                            <DataGridTextColumn Header="Фамилия" Width="Auto" Binding="{Binding LastName}"/>
                            <DataGridTextColumn Header="Имя" Width="Auto" Binding="{Binding FirstName}"/>
                            <DataGridTextColumn Header="Отчество" Width="Auto" Binding="{Binding MiddleName}"/>
                            <DataGridTextColumn Header="Должность" Width="Auto" Binding="{Binding Test_Post.Name}"/>
                            <DataGridTextColumn Header="Статус" Width="Auto" Binding="{Binding Status}"/>
                        </DataGrid.Columns>
                    </DataGrid>
                    <Button Content="Добавить нового&#xD;&#xA;   пользователя" Canvas.Left="426" Canvas.Top="24" Width="100" Click="OpenRegistrationWindowClick"/>

                </Canvas>
            </TabItem>
            <TabItem Header="Заказы">

            </TabItem>
            <TabItem Header="Смены">

            </TabItem>
        </TabControl>
    </Grid>
</Window>