Turned off iCloud Photos and now it won't turn on again

First disconnect the iPhone that is signed in with same Apple ID and password on the same network with Mac , https://support.apple.com/en-in/HT208242

The Mac is having short iCloud storage space buy a new plan from apple https://support.apple.com/en-in/HT201318

You have created multiple libraries , so merge them https://support.apple.com/en-in/HT209528

https://discussions.apple.com/thread/8536957

Then open Mac system preferences > click on Apple ID icon > in the side bar click on iCloud , double click on photos check box , it will prompt to enter admin password , the photos with full resolution will get start syncing in Mac .

Once the process is completed , for your iPhone use iTunes to transfer photos from iPhone to Mac https://support.apple.com/en-in/HT210612

Note : the libraries once merged can again be split , as the iCloud storage space was short you have to merge them to make them as system photo library https://support.apple.com/en-in/HT204414 in Mac photo application

léonie is very knowledgable on these matters, but I still am wondering what is happening with your photos. You temporarily turned off iCloud photo syncing for a month. When you turned it off, you might have received a message saying your photos would be deleted from all your devices they were being synced to and from iCloud. Maybe you didn't pay attention to the message or maybe it wasn't appropriate under your circumstances. In any case, it is possible that your photos are in Recently Deleted pending final deletion after 30 days or so. I have never been clear on whether items in Recently deleted count against iCloud storage. If counted against, it might seem like twice the storage would be required after re-enabling syncing. I am hoping that you have never chosen to optimize storage because that would complicate things. It sort of looks like you did chose optimization because of the message that Your full-resolution photos will not fit on this Mac--which intimates that they are not already there. See Optimise storage space on your Mac https://support.apple.com/en-in/guide/system-information/sysp4ee93ca4/10.14/mac/10.15. If your full-resolution photos are in Recently Deleted, then they can be restored to Photos before re-enabling syncing. Another possibility is that you photos are still in Photos on iCloud, but I doubt it. Have you looked in Photos to see what is there?

One more thing, Amelio. You did not yet tell us, which iCloud storage plan you are using. And why you needed to disable iCloud Photos. As you found out, we need a safety line of free cloud storage, at least the size of the library to be able to toggle iCloud storage on and off. It should not be done without being a necessity, because it is so expensive.

There are two bottlenecks:

1. iCloud sync wants to upload all your photos again, so you need twice the amount of storage in iCloud to be allowed to try to upload. You will not really use twice the amount of cloud storage, but iCloud will not let you try it, if you might exceed your cloud storage.If you do not want to reduce the size of your local library, as you do not know, if your iCloud library really still has all your photos, you could temporarily sign up for more cloud storage - paying for a month for more cloud storage should suffice to be able to upload the library again.
2. And iCloud wants to download the library again from iCloud and estimates that the combined library will have twice the size. This can be avoided by enabling "Optimize Mac Storage". Then the initial syncing will be quicker and need less storage, because Photos will not download the originals, as long as "Optimize Mac Storage" is on.

So, would it be possible to increase your cloud storage for at least a month? then you do not need to delete either your local library nor the cloud version. Without paying for more storage, you have to sarifice one of the libraries - either the older iCloud Library or the current Mac Library. If you decide to erase the current Mac Library, copy it to an external volume to save any photos you delete. If you decide to wipe the iCloud version, download the photos from iCloud to a new, empty library on an external volume, to save the currently availabe iCloud Photos.

When Photos is syncing your library again after you turned it off, it will treat it as a completely new library. It will upload the complete library from your mac to iCloud and merge it into the existing library . This should not create duplicates, because Photos will check for duplicates. But it estimates the required storage in iCloud and on your Mac based on the worst case assumption, that all photos are different on the mac from the ones already in iCloud. It is to be expected, that you will need twice the storage of the current size of the library, if you temporarily disable iCloud and then turn it on again. Your local library should essentially be empty.

To get the library in sync again, I would make a backup copy of your library on an external volume, then delete all older photos, that have not not been modified recently from your library, to make the library small enough, that you can enable iCloud syncing.

You can find the photos you can remove using a smart album like this - searching only for photos that have been imported a long time ago and not edited. Combine the smart rules with Match all: