What studio version are you using and what project type is it?
The problem is most likely cause by the difference in .Net version of the package and your project. If you’re usng Studio 22.10.1-22.10.3 you won’t be able to see .Net4.x packages when you open Windows project. Vice-versa you won’t be able to see the .Net6.x packages when you open them in Windows-Legacy project.
If you’re using marketplace libraries then it’s most likely that you won’t be able to use them with Windows(.Net6.0) projects.
Hi @NayabImran and welcome to the Uipath Forum
This is most likely a compatibility issue.
If the package is developed in Windows Legacy and the current Uipath project is Windows, then the library won’t show in Studio.
Try to create a new project of Windows Legacy type and check again.
Let me know how it goes