Cannot sync MacBook photos to icloud

Hi everyone, I am trying to sync my macbook photos to the icloud. I have a lot of pictures on my phone that are already in the cloud (2023- current) but my laptop is super old and I have pictures from like (2015-2023) that are not on the cloud. How can I sync my photos on the mac app to icloud? When I try to select 'settings' -> 'icloud' in photos I get a message that says "icloud features for photos are only available in the System Photo Library." What does that mean? I also have tried through system preferences -> Apple ID -> Icloud -> turning on 'photos' but I also get a message there that says "Open Photos to choose or create a library to use with iCloud Photos. iCloud Photos can only be used with one library at a time." So frustrating, feels like I'm going around and around. Any tips appreciated!!

Lyss