You are unable to add extension field to a section or pane.
Reproducing the Issue
Go to Adaptation Mode--> Edit Master Layout
In a section or pane, Add Items--> Add field
You can see the field in the list of add fields
When you try to add, it is not visible
The section you are trying to add the field to is a PasteElement. Hence no changes can be made to this section.
Try adding the field to the source section and then copy paste the section to target.
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