Have you made an order with BT, if you haven’t then even if you found a socket, you wouldn’t have service.
If your intention is to order service once you confirm a socket is present ( and one isn’t ) then there are a few possibilities, the flat had a socket that the previous occupant removed , the flat has never had service ( but can have it ) or the flat has never had Openreach type service and that in part is because the flat ‘owners’ or those responsible for its management have never allowed OR to supply service there.
Its probably a good idea to check with the building manager if ‘BT’ are allowed to supply service ( it may be the case that they are not ), if they can, and you call or go online and the address is recognised, then an order can be raised, after that it’s not really your problem.
If it turns out that a previous occupant has done irreparable damage to the original OR ‘line’ then it may be problematic to ever get that ‘line’ restored, given the ‘high rise’ nature of your address, or it could be that the building is ‘tied’ into a non Openreach provider and only that company can provide service as and when it’s requested
