You want to add a custom field in a section but the option to Add Field is not available, but the same is possible in the header of the BO.
In SAP Cloud for Customer RUI.
Reproducing the Issue
Open BO ABC (ABC is a Business Object where you want add the field)
Enter in Adaptation Mode
Click on Add
There is no option to add new Field
This is expected system behavior.
To add a field in a specific section you can add the field in header and move the field in the necessary section.
add field, not visible, RUI, move, new section , KBA , AP-RC-UIF-XTS-HTM , UI Cross Topics HTML , How To