NU1101: Unable to find package iTextSharp. No packages exist with this id in source(s): Connect, e:\users\chenql2\AppData\Local\Programs\UiPath\Studio\Packages, https://gallery.uipath.com/api/v2, https://packages.uipath.com/activities, Local, Official
Oh you already created another topic with the same problem a few minutes ago, here is the thread where you have already several answers: Can anyone tell me how to figure out this problem？Thank you sssssso much~ - #2 by Hiba_B
This happens when you don’t have nuget.org enabled under manage packages option as some packages do require this enablement.
Can you please check under Manage Packages settings window at project level, if all below options are checked along with nuget.org?
And then try again.
Once you do this, and try to install package from Manage packages in studio, it will work.