Unable to delete empty folder in the trash on Mac Pro

Library folders are generally intended to be marked invisible to avoid accidental deletion.

command-shift-period toggles visibility.

There are in fact three Library folders, each of which are meant to remain locked

/Volumes/boot volume/Library

holds all the preferences, caches, and logs for operating system level applications

/Volumes/boot volume/System/Library

holds the same just for the installed operating system.

/Volumes/Users/yourname/Library

holds for that user account the same. Local email storage, and local user account settings for specific Apps are managed here.

If you move any of these three folders into your Trash bin, the system will balk at you. So put them back where you found them. Only delete those items in those folders you know don't belong.

Applications reported by Etrecheck that you know to be non-essential, should be reinstalled, and deleted with Freemacsoft AppCleaner. That will find those files in the Library folders associated with those Apps if those Apps don't have their own Uninstaller.

Outside of that, never trash a Library folder without knowing its source.