Hello Lex,
Sorry for the inconvenience and the delayed response over the weekend. I've investigated the issue and can confirm there is a problem.
I've already notified the development team. This will be addressed in the next update. In the meantime, please replace the JRoot/administrator/components/com_spbooking/models/forms/package.xml file with the file provided in the hidden content below. This should resolve the issue for now.
If this doesn't solve the issue, please reach out and I'll look into it more deeply.
We appreciate your patience and understanding.
Best regards