How can I sync photos with dropbox

You can keep photos in your Dropbox, but not a Photos library. The syncing with Dropbox will corrupt a Photos Library.

Move your photos Library to an external HD that is locally mounted (not a NAS), and is formatted MAcOS Extended (Journaled) if you need to move the Photos Library from your system drive.

You can import photos from a Dropbox folder to Photos, but you cannot keep the originals on Dropbox and reference them there. And you cannot sync the photos in the Dropbox with with your Photos Library.

If you want a cloud based solution for the Photos.app, with the original Photos stored off-site, use iCloud Photo Library (iCloud Photo Library FAQ)

That is the only supported way.