"Photos Library can not be recognised"
This isn't a question but an answer. Today I tried to open Photos on my Mac and that message appeared. I panicked and thought that, while deleting things the other day, I might have deleted my whole gallery and lost all my photos forever. I searched for answers and tried everything they said and nothing.
But I discovered that when you create a new Photolibrary it directly gets saved in Finder>Pictures, and I emptied and reorganised that folder into Documents the other day. So I searched in Documents for my library and found it!! Because I moved it, Photos couldn't find it and therefore couldn't open it.
Thought that it could be helpful for someone!
MacBook Pro 13″, macOS 10.15