You have to use an exit or a Badi to control your requirements, there is no customizing setting for this.
While the requirement for a requisition is almost understandable, restricting PR creation in case the value is less than X is not understandable, as it just causes double processes, because the info that something has to be ordered needs to be transmitted somehow from the requisitioner to the buyer.
A further question is: what is so important in the requisition to make it mandatory?
A buyer can be controlled with a release strategy on the PO.
And you must not forget that there is not necessarily a 1:1 relation between PR and PO.
what if 2 persons wants independent of each other an item with a value of 6000, and the buyer put it together into 1 PO, so he finally has a PO of 12000. Does he then need to ask both to give him a PR? Or will he better create 2 POs to get around this restriction?
We have for example the email address of the requester in the PR, when the invoice is received and the quantity has a difference then it get mailed to the requester for approval. Without PR we could not do this process, hence it is easy - no PO without PR, just 1 process.
Think about if it makes really sense to restrict people from creating PR if the value is less than x.