In PR screen layout, you can change field selection for valuation price. In your system, you  already tick that field to become optional, but it still require input when PR created. You have already check order type and field selection. 

So far, when you create PR, there is always a valuation price - Assumption is that SAP gets the PR valuation price from the current unit cost of the material. 

In your current business process. User are require to input item in PR, even the valuation price of that item is zero/free goods. So, you set field selection valuation price in PR to just optional. But, it didn't work. in SAP help, it is mention that you can set that field to optional or require. 
 

As far as the business processes requirements are concerned, the Valuation Price of PRs must be set as mandatory since this is one of the main characteristics to set the Release Strategy based on which the release codes are to be allocated for on-line release of PRs in SAP. 

In case, this field is optional, once manual PRs are to be maintained, the Release Strategy will not be effective unless end users remember to fill in this Valuation field. 

What SAP can additionally provide you is to set this field to "Display" only (via Transaction Code OMF2). However, it is not recommendable from the business processes point of view. 

You can only enter a PR WITHOUT PRICE if you did not tick Invoice Receipt under Valuation.

Logically, any items that require an INVOICE RECEIPT must have a price.

See also

• Purchase Order IDOC NEU Output Type Configuration
• Blocking The Purchase Order
• Steps to Create Auto PO

SAP MM Books
SAP Material Management Certification, Interview and Configuration Reference Books

SAP Material Management Tips
SAP MM Configuration Tips and Materials Management Discussion Forum

Main Index
SAP ERP Modules, Basis, ABAP and Other IMG Stuff