After disabling iCloud Photo Library, cannot re-enable due to insufficient iCloud Storage

Solution 1:

A suggestions I've seen on other Q&A forums is to use another device with iCloud Photo Library enabled and wipe the Library so there is enough space for all your photos to be re-added.

If you don't have another device with iCloud Photo Library, I believe you can delete the files from General > iCloud > Storage > Manage on an iDevice.

Solution 2:

it's year 2019 and I encountered the same problem on iOS 12.4.

Here is how I solved it just now:

1. Goto Apple ID > iCloud > Photos;
2. Turn on iCloud Photos. There will be a popup prompting insufficient icloud space;
3. Press the "upgrade plan" (or something similar, I can't remember the exact words) in the popup, then a new window shows with several icloud plans;
4. Note that there is a button at the bottom saying "continue without purchasing". Press it;
5. Then, the iCloud Photos option should be on, and the Optimize iPhone Storage is automatically chosen. You can turn it back to Download and Keep Originals if you like.

Apologize for not showing screenshots but I hope the above message could help others who run into the same issue in the future. ;)