I installed Nintex on SharePoint online and created WF. When I ran the WF, it was stuck on 'Start' and didn't run eventually. Is there any other configuration that I'm missing? (I use the on premise version for the last 5 years)
Try to look at sharepoint ULS logs. Maybe there you will see the problem.
On the workflow status page, click on the blue 'i' icon to get details on the state or error of the workflow. Most likely there is an issue with the workflow configuration or data it is trying to use that is causing the workflow to error. What do you see when you click on the blue information icon?
Retrieving data ...