You must use a virtual device which is both a switch and a contact sensor. The code for one is at the FAQ link that I already gave you. You will be able to use the virtual sensor as the trigger for the Alexa routine. And you will be able to turn it on from smartthings by using the virtual switch part of it. Turning on the switch makes the sensor look like it opened.
All of this is explained in the FAQ, so just read it carefully and follow each step exactly in the order given. 
The discussion of the specific error that you had is in the FAQ in the forum, but just following the process in the community – created wiki should avoid that error. Don’t skip any steps even if you think you have already done them, they have to be done in a specific order.
You can read discussion of the error in the link I already gave you: