The library isn't compatible with newer swift versions

[orre1996]

Seems like some packages needs to be updated

[alexanderjordanbaker]

This library only supports Swift on Server, not devices like an iPhone. We recommend using StoreKit 2 on device.

[orre1996]

Ohhh that makes sense, sorry about that.
Unfortunately promotional offers can't be done with only StoreKit 2, you need to create the signature on the server from what I've read. Unless I've completely missed something. And for the JavaScript code I get an error saying that applicationUsername.toLowerCase is not a function when I try to create the signature

[alexanderjordanbaker]

So yes, to create a promotional offer you need a server which has access to your app's private signing key (Another reason this can't be done on device, as the app wouldn't have access to this key). We can track that issue on the issue you created on the node library.