Audiobook issues iPhone 8 and iwatch3

my wife has an iwatch3 and iphone8. she has downloaded an audiobook on her phone. when she opens the watch app on her phone and clicks on the audiobooks link - there is no option to ‘add audiobook’. There is no link to her library at all. It just has a big link to the audiobook store. So how can she add her audiobook to then sync on her phone? Both watch and iPhone are updated to iOS 6 and 13 respectively