Вы не указали имя для поля выбора, вам может помочь следующее
extends ../layout
block content
h1= title
form( method="POST" action="/newloan")
p
label(for='book_id') Book
select#book_id(name="book_id")
each book in books
option(value=book.id)= book.title
p
label(for='patron_id') Patron
select#patron_id(name="patron_id")
each patron in patrons
option(value=patron.id) #{patron.first_name} #{patron.last_name}
p
label(for='loaned_on') Loaned on:
input#loaned_on(type='date' name="loaned_on" value=loan.loaned_on)
p
label(for='return_by') Return by:
input#return_by(type='date' name="return_by" value=loan.return_by)
p
input(type='submit' value='Create New Loan')