Based on this old thread :
When you are trying to query logs such as activity logs and Perf logs from log analytics workspace , the queries doesn't return the results. Kindly note you need to enable diagnostic settings for activity logs to send to logs to log analytics workspace.
To enable diagnostic logs for Activity logs : Go to any resource -> Click on Activity Log Tab on left pane -> Click on Export Activity Logs -> Click on "Add Diagnostic Settings" link -> Enter the diagnostic name , Select the Logs , choose the log analytic workspace where you want to query your data. For more information, check this document.