"tpAttachNowait" is a parameter and it holds for all widgets that I use. "Visible" is a property. The latter may not exist for all objects, but for simple buttons (at least recognised by TestPartner as "Button") it does. The message that you "some times" get, does not concern the tpAttachNowait parameter but the property you call.
Make sure that this property is defined for your object (is it proposed by TestPartner when you type "."? You may also hit F2 to see all objects available together with their methods and properties.
You may also check whether an object does Exist before checking if it is Visible (if it does not exist then it is not visible, right?)