You are not able to add custom fields in custom pane/tab.
Reproducing the Issue
- Go to the Sales work center.
- Go to the Opportunity view.
- Open any opportunity and navigato to custom pane/tab.
- Select Adapt -> Edit Master Layout.
- Try to add custom field to a section.
- It is not possible to add any field.
This is the expected behavior of the SAP Hybris Cloud for Customer.
You can create the custom field at Header of the opportunity and drag it to the custom tab.
KBA , LOD-CRM , Cloud for Customer , How To