Am trying to install webshop to my site but I keep on getting errors
Am using this command “bench --site my site.com install webshop”
Unfortunately for me I received no guidance but had to figure it out myself, after 48 hrs of tiresome troubleshooting and seeking for solutions.
Fortunately for you, I figured out a solution that worked for me 
You need to install the payments app first with ‘bench --site site.com install-app payments’.
Next, install the webshop module. It should be successful. I will be requesting for this to be documented under the module itself.
Also, keep in mind that this (somehow) does not work on the bitnami edition (permission issues I think) , but only self hosted on Ubuntu server.
Hope this helps you out.