Photos to Flickr
How do I upload pictures from my Photos library to Flickr?
Photos 4.0 on macOS 10.14 Mojave does no longer support the upload to social media like Flickr. Apple removed the sharing extensions for Flickr and FaceBook.
To upload to Flickr use the Flickr webpage and start the upload from there. You can either use the Media Browser on the web page or export your photos from Photos with "File > Export" to a folder on your Desktop, then upload from this folder.