Correct, you select a build, but then the option to update the IAP should be an option at the bottom of the distribution page. But that option isn't there to add the IAP.

Correct, my wording is bad there. I'm not trying to connect it to my build. But I select a build, and then there should also be an option to attach the IAP to the new distribution in App Store Connect, but it isn't there. I have seen it once before, and I have attached an IAP, but that app version was rejected and the IAP was returned and now my current version was approved, but my IAP is rejected and they want me to add it, but the option isn't there to do so.

This has not been resolved.