I tried to create a new iCloud account and on that one I had no problems on the same computer. However, in my usual iCloud account I can’t synchronize iCloud Drive in Finder, even after trying it on another user and after a clean install of El Capitan on the same computer.
So, I definitely considered that it was a problem with my iCloud account and contacted Apple.
Apple technical support made several checks on my computer even with captures of information that they took to analyze the problem and scaled the issue to higher departments to see if they found a solution, thus reaching the engineering department of iCloud drive. There they pointed out that there are currently two versions of the iCloud system, one of them is more updated than the other. Some accounts already have the updated version of the iCloud system and others do not. Those with the updated version are NO LONGER COMPATIBLE WITH THE CAPTAIN. The accounts that maintain the old version of iCloud, continue to run smoothly. This is something that the user can't seem to choose, so iCloud may at some time to break the compatibility with El Capitan without prior notice. And since El Capitan is a system considered obsolete by Apple, there is no possible solution.
That said, when iCloud Drive stops working on an account with El Capitan, synchronization with iBooks and Safari bookmarks also stops working.