'Why richtextblock not supported?

I'm trying to add richtextblock to my project in Xaml .NETFrameWork 4.7

and I'm getting errors-

print screen

It's important to note that I can not find it in the toolbox, should it be there?

I googled I did not find any place that says something needs to be installed, I would love for you to help me

Thank you

wpf


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