You need to review your condo documents to see how they classify the balcony. It may be listed as your property or common area with exclusive use (of you) or Master Element property. Condo docs and management in Thailand is poor in this respect in many condo buildings. Typically (in the USA) you only own and are responsible for internal wall to wall and floor to ceiling and any piping that is your exclusive use meaning serving you. Piping that is enters and exits your condo and is shared, main drain lines, fire safety, main gas supply etc are common elements before and after they enter or exit your condo. Balconies can be common element for your exclusive use, which gets into a grey area or Master element like windows, roof and exterior walls. Based on your description of it being a balcony floor drain it should be the condo master entity and they are responsible but no guarantees.