[AppInitio]

Sure. As the alert says: Quit Photos. PhotoSort gets file size and other data from the same database that powers Apple's Photos app, so (in order to preserve its integrity) Photos 'locks' the database when it's using it. Closing Photos will solve the issue. Please confirm once it's resolved (or if you still can't).

[flxfxp]

Apologies if I was not clear: I obviously followed the prompt and quit the app several times before deciding to reply to your message. It's still giving me the same message.

[AppInitio]

Sorry about this: Very unusual. If the app can't access the library, presumably the 3+3 thumbnails on Home screen and the Top-30 report in View menu are also blank? Please reboot the Mac and try; if even that doesn't help please email us via the app mentioning your macOS version, so we can troubleshoot step by step.

[flxfxp]

Seems like a pending OSX update was the case; prior to that I rebooted twice and it wasn't working, after updating OSX it worked :-)

[AppInitio]

Yay! If you find the app useful, please do rate it on the App Store - that's the only way indie developers can gain some visibility. Thanks.