John Haley1 wrote: How do I download the photos library in iCloud to the Pictures folder in my home folder on my Mac?
If I get your meaning, that's not exactly how iCloud Photos works. You don't download a Library. But it may be that I'm not understanding what you mean.
Where is your Library? If you look at iCloud Drive in your Finder's sidebar, do you see the Photos Library? If so, that's not where it's supposed to be. If this is the case, then that would explain why the Library can't open. In that case, try dragging the Library back to the Pictures folder and see if it begins working.
iCloud Drive and iCloud Photos are two different things. For pictures, each device has it's own Photos Library on the device. Your iPhone has a Photos Library; your Mac has a Photos Library; iCloud has a Photos Library. When you choose to use iCloud Photos, then that keeps all those Libraries the same. The Mac Library, usually named Photos Library.photoslibrary is normally kept in the Pictures folder. It can be other places, but it must be kept on a local drive connected by a wire to the Mac-- the Library can not be kept on a network or "in the cloud." If you see a Photos Library in the iCloud Drive, it should have a little cloud icon next to it with a line through it indicating that it's not really in the cloud-- the Mac wouldn't let it be put there. So, if you drag it back to Pictures, and if nothing bad happened, it will be back where it belongs, and it should work.
If this isn't what you meant, then what will work is to make a new, empty Library, make it the System Library, and connect it to iCloud Photos through the Settings. The the new library will fill with your pictures from iCloud Photos.
Let us know more details about what's going on....