Hello,
There is a purchasing policy which governs whether PR will be automatically converted to a PO on approval.
Please review the policy which is set under Purchasing policies >> Purchase order creation and demand consolidation >> Manual Purchase order creation section.

If Automatically create purchase orders is checked then PO's will be created automatically when the PR is approved, Provide the vendor info is filled in the PR.
Thanks, Kaushik
This post is my own opinion and does not necessarily reflect the opinion or view of my company or Microsoft.
P.S:- Please mark the answer as verified if it helps you in resolving the issue.