The ‘PRINT_BIBLIOGRAPHY’ keyword specifies where the bibliography should be printed (note the colon):
The bibliography printed by the LaTeX-based export processors “bibtex”, “natbib” and “biblatex” has a chapter or section heading by default, while the “basic” and “csl” processors print the list of bibliography entries without a heading.
A document may contain more than one ‘PRINT_BIBLIOGRAPHY’ keywords. Each of the keywords will trigger printing the bibliography.
The keywords can be used with or without additional options. Options can be used, for example, to print only entries that belong to a certain category or to control formatting. The set of supported ‘PRINT_BIBLIOGRAPHY’ options and their interpretation varies between the different citation export processors. Some export processors do not support passing options.