How to create a validation rules to control the Purchase Price cannot more than Last Purchase Price?
Please refer How to Access Customization Centre.
Step 1 - Validation > Rules > Add… > RuleCriteria
Step 3 - Set as below:
Id: Rule name (PurchasePriceNotMoreThanLastPurchasePrice)
Target Type: Which transaction apply rules (QnEBS.BO.Purchase.PurchaseInvoiceDetails)
Criteria: Rules/Conditions ([UnitPrice] >= [Stock.PurchasePrice])
Step 4 - Save and re-login QNE