You want to add a custom field in a section but the option New Field is not available, but the same is possible in the header of the BO.
In SAP Cloud for Customer HTML5 UI.
Reproducing the Issue
Open BO ABC (ABC is a Business Object where you want add the field)
Enter in Adaptation Mode
Click on Add Field
New Field is not available
This is a system expected behavior. Currently add field is some sections is not supported.
To add a field in a specific section you can add the field in header then drag and drop the field in the necessary section.
KBA , AP-RC-UIF-XTS-HTM , UI Cross Topics HTML , How To