You have added a Mashup or Embedded Component at Workcenter View, however when trying to set the Expand / Collapse functionality it's not working as expected.
SAP Cloud for Customer
The expand / collapse button will not appear as this functionality is not supported at workcenter view. It is supported only in Thing Inspector (TI) screens.
If you need this functionality one option is consider to move the UI element to TI screen instead of Workcenter View.
expand, collapse, mashup, EC , KBA , AP-RC-UIF , C4C UI Framework , AP-RC-UIF-FLX , Page Layout & Key User Configuration , How To