block opportunity creation without contacts

You cannot block Opportunity creation based on a related record existing unless it's a relate field rather than a M-M relationship like the one between Opportunities and Contacts. The Opportunity has to exist before you can add a contact.

One possible option, though, is to add a Relate field to Contacts on the Opportunity main panel and make the field required, you can call it "Primary Contact".

Then, if you want to, you can add some custom logic that will also add a relationship between that contact and the opportunity in the original relationship record so the Contacts subpanel would show the primary contact there as well and any reporting based on that relationship would include them in as well.

FrancescaS

That's a great answer and workaround Francesca. That's what I was looking for. Thanks heaps!

If you look to the Opportunity-RLI relationship you find a waterfall relationship from Opps to RLIs which does not allow to save an Opp without an RLI. So, it is possible to extend the Opportunity create view JS controller by the code for Contacts also, BUT this creates highly customized code which must be checked with each upgrade.

That's why I think the proposal from Francesca Shiekh is a much better way to solve this requirement.