How to turn on iCloud Photo Library
It keeps asking .... your password is required to enable iCloud Photo Library ... says open Apple ID
in system preferences to turn on iCloud photo library
iMac 21.5″, macOS 10.15
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You have to be signed into your iCloud account in the System/Apple ID preference pane. Then click on iCloud in the left hand sidebar and then check Photos in the right hand pane:
Next open your Photos library and make sure it's designated the System Photo Library;
Lastly go to the iCloud preference pane in Photos and check the iCloud Photos Library option.