Photos settings iCloud button displays "only available in System Photo Library"

You have to make the Photos Library your system Photos Library, before you can turn on iCloud Photos.

Then turn on iCloud Photos.

Have you been using iCloud Photos on the old Mac? And has "Optimize Mac Storage" been enabled? Then the copy of the Photos Library will be incomplete. With "Optimize Mac Storage" enabled iCloud may remove original image files and keep them only in iCloud, if the Mac is running low on storage. When you move a library to a new location, the optimized version will be showing as blank boxes, because the originals are missing and Photos will remove the incomplete items, when you disable iCloud Photos.

I hope, all your Photos are still in iCloud? You need the originals in iCloud to restore the missing originals.

If your photos are still in iCloud Photos, you do not really need the copy of your Photos Library. You could simply start over with a new, empty Photos Library (Create additional photo libraries in Photos on Mac – Apple Support (UK)), make this your System Photos Library, then enable iCloud Photos and and let the library sync with iCloud. Your complete library should sync back from iCloud and this will be faster than starting with the copy of your library. With an empty library Photos will not need to upload the library to iCloud Photos and merge it into your existing library. It will only need to download the library and thus save a lot of time.

The only thing missing will be any projects you created, because the projects are not syncing with iCloud Photos. To save your projects, keep the copy of your library.

Some un-asked-for advice: Since you have "Download originals" checked, you can backup your Photos Library by copying it to an external drive, or you can use Time Machine for the backups to happen without your thinking about it. iCloud synchronizes your Library, but if you inadvertently delete pictures on your Mac, they are deleted in iCloud as well-- so a backup is a really good idea.

You're welcome 😊

I'm glad t worked for you.