Note that PhotoStructure for Docker needs correct PUID and PGID values: see https://photostructure.com/server/photostructure-for-docker/#running-photostructure-for-docker-as-a-non-root-user for more details–I suspect your library is not owned by uid 1026. I’ll add who actually owns the library to the health check message right now.
One nasty bit about Synology’s UI is that they seem to hide the user id (at least I couldn’t find it!)–I had to ssh into my NAS as the user that owns my photos, and run id to find the value to set PUID and PGID to.