Users in a group with Full Control on the list get access errors when trying to stop workflows created in SharePoint Designer that I started. I have Full Control access as well. I have no problem stopping workflows started by anyone.
Whether you have permission on the site?
If we want to end the workflow for the items, we need at least Edit permission on the site.
Here is a MS link on the subject of permissions - as you see, a group e.g. 'Edit' is comprised of a number of individual permissions, including 'Manage Lists'. By adding a user to the Edit group, they should be able to stop the workflow.