Well, yeah… theres no object for the prompt to connect to unless as @Dyllon3761 said, you set a PrimaryPart on the model. I mean, it’s either an actual bug or the documentation just needs to be fixed.
Thanks for bringing this up. I looked into it, and using a Model for the parent of a ProximityPrompt does require PrimaryPart to be set. I see that this info is missing from the documentation - we will update that.
The decision to not use Pivot at the moment is a performance consideration. In some cases, that needs to traverse through all child parts of the model in order to calculate a bounding box and find a center.
We may decide to relax this requirement in the future, but at the moment this is working as intended - you will need to set PrimaryPart to make it work. Thanks!