I have a user who can't see the "Workflow" tab, but just sees the tabs for Dashboard, Sheets, and Reports. Other users can see the Workflow tab. I don't see anything in the user setup that would be causing this.
Only users who have access to a parent level and have the "Approve Levels" permission will see the Workflow tab. Users who do not have access to a parent level or do not have the "Approve Levels" permission will go to Sheets > Overview to submit their work through Workflow. If you have access to a parent level, have permission to approve levels, and still are unable to see the Workflow tab, make sure you are viewing a non-actuals version.