In Sharepoint Online you may face with the problem when will try to create workflow in Sharepoint Designer 2013: only “Sharepoint 2010 workflow” will be available in Platform Type dropdown list, while new “Sharepoint 2013 workflow” won’t be there:
The problem will be more unclear after you will check in admin center’s settings page that “Block Sharepoint 2013 workflows” checkbox is unchecked:
In order to enable “Sharepoint 2013 workflows” in Office 365 go to Site settings > Workflow settings:
“Workflow Health” in parenthesis is actual a link (very hidden link which looks like text. This is for sure should be fixed by MS somehow). Click on it and you will see the following page:
On this page click Activate button. After that when you will reconnect to the site in Sharepoint Designer you should see “Sharepoint 2013 workflow” option in the list of available platforms:
Please note that this configuration should be made per SPWeb. I.e. if you activated it only on the root site, it won’t be available on the sub sites. You will need to activate it on sub sites where you need Sharepoint 2013 workflow separately. Hope that this information will help you.