I keep getting this error message when I try to click \"Test web app for your latest code.\" in the Publish dialog box.
But I haven\'t defined any function called do
As Serge insas said, you must first implement the function doGet(e) as described in Google's documentation.
However, after you implement it, make sure to save a new version of your project (File > Manage versions... and then save the new version) and then deploy the new version of your web app. Else you will continue to get the "doGet not implemented" error from Google. This is because it is still using the old version of your application, before you implemented doGet.