learn.microsoft.com/.../devenv-report-ext-object
The report layout of an existing report can't be extended, only substituted. To use an existing report as a starting point, you can download the layout from Business Central and include it in the extension project.
Layouts that are included in a report extension will show up in Business Central as built-in layouts. The layout in a report extension will not automatically be used when the report extension is deployed. To use the report extension layout, go to the Report Layout Selection page in Business Central, make sure to choose to use a built-in layout in the Selected layout field, and then choose the layout for the report in question from the Custom Layout Description drop-down box.
You can automatically update a report layout from the error message that appears when you run the report by choosing the Yes button on the error message. Or, in advance of running reports, you can update specific report layouts or all custom report layouts that might be affected by dataset changes.
Update Custom Report Layouts - Business Central | Microsoft Learn
