The Convert tracking code (also sometimes referred to as project code snippet) is not installed on your website.
The Convert tracking code is installed, but not from the correct project. Each project in Convert Experiments has it's own tracking code. Make sure you add the code that belongs to the Project in which you created your Experiment.
Make sure the URL(s) in Active Websites in your Project Settings matches the website you installed the Convert tracking code on.
Visual Editor URL
Preview is showing a different page than the target defined in the Site Area. That would be because the preview is based on the original URL you used when you first created the experiment. After creating the experiment, the Visual Editor URL can be modified by going to your Experience Summary, and under "Experiment", clicking the edit icon.
Conflict with Other Experiments
You have another experiment which has the same Site Area conditions and also has the status of Active, Completed or Paused.
This is because Live Preview ignores Audience conditions. For instance, if you have an experiment with an Audience defined for mobile users, and another experiment with an Audience defined for desktop users, and both with the same Site Area conditions, you could also be included in the other experiment when you do a Live Preview on either of these experiments.
Problems with Variation Code
If you have checked all of the above and still cannot resolve the issue, the problem is likely related to the Variation code:
If you made changes to the Variation using the Visual Editor functionality, it is possible that there is a conflict with the code on your website and you may need help from a developer on your team.
If you manually wrote the variation code, it is possible that there is an error there. Any error in the code would cause the code to fail to execute, therefore not rendering your variation.
When you use a Preview Link, we append parameters that look like this: ?convert_action=convert_vpreview&convert_e=121212121&convert_v=100111234. Sometimes those parameters might be removed by your host or server. You can notice this if you look at the address bar in your browser. If you do not see these parameters, you are not in Preview mode. In that case, see Additional Information below.