You will not find a single Venezuelan lawyer expert in Constitutional Law that thinks the National Constituent Assembly is legal. It was invoked by Maduro without a Yes/No referendum, which is required by law (http://www.minci.gob.ve/wp-content/uploads/2011/04/CONSTITUCION.pdf), and the company responsible for the technology behind our elections for over 10 years said the results were tampered with: http://www.smartmatic.com/news/article/smartmatic-statement-on-the-recent-constituent-assembly-election-in-venezuela/

It's quite simple. The petro is new issuance of debt, Venezuela's constitution explicitly requires new debt to be approved by Parliament (not by Constituent Assemblies), hence the petro is illegal.