There are known issues with the latest version of Unity IAP 1.22 and Unity 2019 that we are looking into. The import process may fail, and you may see build errors and missing references. Apologies on the inconvenience, this is something we are working on at high priority. Some users have mentioned that they got it to work by copying folders from a working Unity 2018 project, but we haven't tested this yet. Another known issue is a possible requirement for Unity 2019 itself to be installed to a path with no spaces in it for the API updater to properly work ("C:\Unity" would be OK, "C:\Program Files\Unity" would not). I will provide updates on this issue in this thread.