GI on Mobile app does not show the "+" sign to add new entry

You will need to add the GI to the mobile screen which you can do by following this article because you will need to add the Add Action to the mobile screen. You can turn of the Display on mobile app on the GI

https://www.acumatica.com/blog/acumatica-mobile-framework-simple-customization-to-display-codes-in-the-mobile-app/

If this is already in the mobile app then edit the screen and add the bit below in bold.

Where you are adding the GI screen you will need to add the action highlighted in bold. My example is using the Customer GI.

add screen AR3030PL {
  add container "Result" {
    fieldsToShow = 6
    listActionsToExpand = 1
    containerActionsToExpand = 1
    add field "DisplayName"
    add field "CustomerID"
    add field "CustomerName"
    add field "CustomerClass"
    add field "Country"
    add field "City"
    add containerAction "editDetail" {
      behavior = Open
      redirect = True
    }
    add containerAction "insert" {
      icon = "system://Plus"
      behavior = Create
      redirect = True
    }
    }
    }