Okay, it seems you are trying to WebSet directly on the object definition, as if it were a Set. If you do that, you will get that error. In order to accomplish what you want to do, you can augment the OnBeforeShow (as pointed by other users) of the view where the control is defined and WebSet the psTooltip to whatever value you want. This will work fine for forms, checkboxes, radios,...

However, if you need to do that on your cWebMenuItem, you need to do it before any view is loaded as it is defined in the WebApp menu. For this, I think a good event is OnChangeRights. This is an example:

Procedure OnChangeRights
    String sMyCustomTooltip
    Forward Send OnChangeRights

    // Obtain your desired value here
    Get UserName to sMyCustomTooltip
    // WebSet the tooltip of your menu item
    WebSet psTooltip of oWebMenuItemUser to sMyCustomTooltip