XAML – The property ‘Content’ is set more than once

By adding your text description to your TabItem you added Content then when you added the Canvas you added an additional item of Content which is not allowed for the TabItem. You need to use a Control that can hold a collection of Children such as Canvas, Grid, StackPanel etc. Try something like this.

<TabControl Grid.Row="1">
    <TabItem Header="Document Flow">
        <Canvas>
            <TextBlock>
                This is where the outline of the entire document will be placed.
            </TextBlock>
        </Canvas>
    </TabItem>
    <TabItem Header="Preview">
        This is where the preview will be drawn to screen.
    </TabItem>
    <TabItem Header="Resources">
        This is where the resources { graphic files, fonts, data files }
    </TabItem>
    <TabItem Header="Code Library">
        This is where the user can save re-usable bits of code. Useful when adding intel barcodes or Address blocks etc...
    </TabItem>
</TabControl>