Hi Ridma,
If you are referring to the solution provided by Microsoft:
Access the default insights workbook for your Application Gateway resource. All existing insights supported by the Application Gateway analytics solution will be already present in the workbook. You can add custom visualizations based on metric and log data.
Migrate from the Azure Gateway analytics solution to Azure Monitor workbooks
You should be following the solution provided: "ensure that diagnostics settings are enabled to store logs in a Log Analytics workspace. If it's already configured, the Azure Monitor Network Insights workbook will be able to consume data from the same location. No more changes are required."
Once you see the logs coming in, you are then free to delete the default insights workbook for your Application Gateway resource.
If this is helpful please accept answer.