I am trying to create a workflow to auto create subfolders when a new folder is created but I don't have SharePoint Designer. Is this possible?
Unfortunately, it's not possible, the available reusable list workflow like
- Publishing Approval workflow.
- Approval workflow.
- Collect Feed Back ...etc
So at least you should have a SharePoint designer or visual studio to achieve your goal.
Once you decide to install SharePoint Designer, you can create a subfolder via SharePoint Designer workflow as mentioned at Sharepoint Designer subfolder workflow in 11 simple steps