The UserId allows the identification of every user within your game or virtual world. You are the one creating them, make sure to choose something unique and immuable. You can for example use the UserID of the distribution's platform, such as the SteamID of your users.
When importing SDK core bundle, I have those errors:
Failed to update package com.kinetix.coreweb2 releases. Error: EBUSY: resource busy or locked, rmdir 'D:\Projects\KinetixMoji2\Library\PackageCache\.tmp-20624-BNH1B2bL6Box\clone'
Failed to update package com.kinetix.uicommon releases. Error: EBUSY: resource busy or locked, rmdir 'D:\Projects\KinetixMoji2\Library\PackageCache\.tmp-20624-esID5FRAJ3t5\clone'
Failed to update package com.kinetix.uiemotewheel releases. Error: EBUSY: resource busy or locked, rmdir 'D:\Projects\KinetixMoji2\Library\PackageCache\.tmp-20624-P4vQY7Rf05rL\clone'
This error occurs when your PackageCache folder is read by other softwares than Unity. You can try to close your IDE (Visual Studio/Rider/etc...) and then download the packages again through the Kinetix Package Manager.