Problemi importazione NAS - Lightroom

You fell into a very common trap caused by Adobe's terrible naming of their products. The Lightroom version you are using CANNOT use local storage such as a NAS. It stores all images in the cloud. If you use the cloud version of Lightroom (confusingly called Lightroom Desktop) and import images into it, it will first copy them all to you built-in hard drive, and then upload all images to the cloud before deleting the local copies. If your internal hard disk is not big enough to hold all the images, this process will fail. 

The actual newest version of Lightroom as you knew it before is called Lightroom Classic. It is constantly updated and has all the editing features the cloud version has and many more features that the cloud version does not have (it is able to print images for example, which the cloud version cannot do). It does not sync all your images to the cloud and relies on local (or NAS) storage of your images. My guess is that this is what you actually want. The cloud version of Lightroom you are using is actually a port of the mobile operating system version of Lightroom (i.e. for Android or iOS) to desktop operating systems. It is pretty neat that it allows you to have the same images on every device but it is completely built around everything being in the cloud.

Please tell us more about your environment. Especially of your free disk space.