Hello linle,
I would like to clarify for which kind of site the workflow could not be created. And, could you create workflows for the lists/libraries of that site with SharePoint Designer 2010?
You can refer to the following steps to narrow down the issue.
1.Try to create a workflow for the site from SharePoint Online.
2.Try to create workflows for the lists/libraries inside of that site.
3.Save that site as a template.
•Login to your SharePoint Online, and locate that site.
•Click Site Actions->Site Settings->Save site as template.
•Type the File name, Template name, check the Include Content, and click OK.
4.Create a new site with this template.
•Go to your Team Site, or click Go to top level site settings.
•Click Solutions under Galleries.
•Select the newly created template, and click Activate in the top ribbon, and click Activate.
•Click Site Actions->New Site->Browse All.
•Select the newly created template to create a new site.
•Try to add a workflow for this site.
If that site was created with a custom solution, and the workflow could not be added to it, in this case, based on my experience, the issue would be related to the custom solution.
Regards,
Cherry Wang