When creating a new table (navigation: System Definition > Tables), selecting the 'Extensible' check box on the form and saving, the check box goes back to 'uncheck'. It can only be set to 'true' through the list view.
The issue occurs when the 'Extensible' field is available 2 times on the Table form. One time at the header level and one time on one of the tabs (usually 'Controls'). When the same field is present multiple times on a form with different values (checked and unchecked in this case), the platform rejects the save as this causes a conflict.
Modify the form layout so that the 'Extensible' field is displayed one time only. Proceed as follows:
- On the Table form, open the context menu (with the "hamburger" menu or right-click on the header bar) and select the option: Configure > Form Layout
- Select the Section you want to modify: 'Table' or 'Controls' (assuming that the field is located at the header level and on the 'Controls' tab).
- Remove the 'Extensible' field from the 'Selected' section.
- Click on 'Save'.
This has also been reported in Problem PRB094076.