I have set two variants ( two data sources for content testing) but it's not showing preview in "preview and start test" screen. Also when i click "start test" button, its keep on processing and never complete.
We encountered this issues earlier on Sitecore 9.0.2 that it was neither showing the previews nor saving the screen shots. Do you see any errors on the logs file? We had the following exception in logs:
16:13:57 ERROR [Content Testing]: Cannot find PhantomJS executable at 'C:\inetpub\wwwroot\MYWEBSITE\App_Data/tools/phantomjs/phantomjs.exe'. Aborting screenshot generation.
If you see the above exception, please see if you have
phantomjs.exe placed at the configured location. If not, copy that from a vanilla Sitecore installation package and test it again.