You have created a custom EC and there is no New button to create extension field.
Reproducing the Issue
Go to work center eg: Customers.
Click on Accoutns View.
Navigate to custom facet.
Click on Adapt -> Edit Master Layout.
There is no option to create extension field.
Anchros are not configured correctly.
To add extension field or to do property changes to a field, first anchors must be defined at pane level.
And for creation of extension fields, in Anchor configuration, repsective BO node and data field must be present. If this is not configured, creation of extension field in this pane is not supported.
KBA , data field , new extension field , extensil field , extension field , anchors , bo node anchor , BC-DWB-REP-EXT , Extensibility , Product Enhancement