“Set ItemID − To display the search result, itemID is assigned by selecting menu item.”
That why you need to create a SEARCH RESULT MENU ITEM to display the search result for the search form particle.
Take a look https://joomlead.com/g5/docs/search-particle-documentation/ and https://docs.joomla.org/Help39:Menus_Menu_Item_Search_Results
You can create a hidden menu then add new menu item with menu item type -> Search -> Search Form or Search Result