You need to configure this as MM Configuration person and this is part of MM Consultant. Go to IMG, SPRO
Material Management-->Purchasing-->Purchase Requisition-->Release strategies for Purchase Requisition.
For business workflow linkage use txn "SWETYPV" and connect the business object to the Workflow template.
Once MM Configuration steps are done then maintain Task ID 00007986 in IMG Settings for Approve Requisitions App as described in the help link that you have.
Once all settings are done first see if you are getting work item to approve in SBWP business inbox to approve as a approver and then check FIORI App.
thanks
Ashish