From what you posted; I assume you are converting a Windows-Legacy project into a Windows compatibility project.
However, the Microsoft.Activities.Extension package still supports only .net 4.6.1.
Please view the following for replacing these activities: 
[FirstAid] Migration to Windows target Framework - Missing Microsoft.Activities.Extensions package - Dictionary Activities - News / Vote on Tutorials - UiPath Community Forum
OR, convert the project back to Windows-Legacy, in which case this dependency will work.
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