The “add list” operator EXPECTS A LIST. You are not adding a list. You are adding an item.
Use “add item” to push a single item onto your list.
Alternatively, append :make list to your right hand item to turn it from a single item into a list with 1 item in it.
(I get that Bubble nomenclature can be confusing. To clarify further: When you have a list and you want to add more items of that list’s type to the list, you have two options:
Push a single item onto the list. This operation is called “add item” (meaning “add a single item to the list in the left”). The item added must not be a list, but a single object of the target list’s type. (If your right hand thing is a list, you can still use it, but you must select just one item to be pushed [you can do this with :first item, :last item or:item # operators]).
Push multiple items at once onto the list: This operation is called “add list” (meaning “add a list of items to the left hand list” NOT “add something/anything to the list on the left”). The item(s) to be added must be a list (an array). It does not matter how many items are IN the list, but it must be a list. A list CAN have zero or 1 or more than 1 item in it. But a single object by itself is NOT a list (an array).
To convert a single item to a list, you can do :make list in it. The single item becomes a one-item LIST of the object’s type.