Hey, I've tried to migrate my catalog from LR classic to LR. The Only Problem is that in the process it shows me that I have not enough cloud storage. But I only have 5 Thousand Pictures with Smart Preview. And my Smart Preview Folder that is in the catalog folder is only 5Gb large. So, what's the problem or did I miss understand something I have 100GB of Cloud storage
Lightroom stores the originals online. The cloud space that you'll need is the space that your originals take (plus the space for edits and metadata), not the space that your catalog and (smart) previews take.
You're probabvly confusing Lightroom and Lightroom Classic's sync mechanisms. There is a lot of confusing info online. Classic will only upload smart previews to the cloud. Those smart previews actually do not count towards your online storage. Lightroom Cloudy as Johan notes has to store the actual originals online. For 5000 raw images which are typically 25-50 MB each, you need close to 200 GB of cloud storage.