Google Photos limits each connected session to about one hour. There's no warning inside Google Photos when time is running low, so if you're still selecting photos when the hour ends, the upload will fail once you finish — even though everything still looks normal on screen.
To avoid losing your selections:
Create an album in Google Photos first with just the photos you want in your book
In Chatbooks, connect to Google Photos and select that album instead of browsing your full library
This keeps your selection quick, so you finish well within the one-hour window
If an upload does fail, reconnect to Google Photos and try again using the album method above.