I know this is a old forum quesiton but also want to add additional information for other folks who may come across an issue I encountered. In the activities panel I was not able to locate Open Application or Open Browser, following the UIPath Tutorials it instructed that I use these activities but I did not have a selection avaiable. I installed the UI Automation package and still could not locate these activities.
Recently I learned you can switch between Modern activities and Classis activities. Modern activities include the Use Application/Browser window whereas Classic acitivites include OPenApplication, Open browser, attach Window, Attach Browser etc.)
I am using for UiPath Studio version 2021.2.2-beta.44 Community LIcense.
Here is how you switch between Modern and Classic
From your project go to the Activities panel
Right above the Search activities field, select the filter icon
Then select ‘Show Classic’
Now you can search Classic activities like Open Browser.