Apparently network boot was deprecated in version 19 and subsequently removed in version 20:
(patch notes for version 19.x.y) https://kb.parallels.com/en/129860
Version 19 is still available for download (select version 19 from the dropdown), and soon will be "legacy" where it will be hopefully available for download here:
(list of legacy downloads) https://kb.parallels.com/5802
