With NBN construction there’s 2 components, the construction of the connections that get the line to the point of use, such as an apartment in a building, and the construction of the network in the street. What’s been happening is the connection to point the of use is built first and the network lags behind.
This is probably what’s happened here although it seems odd to tell you your building is not ready while the apartment is because they are connected on the same haul in. It may be due to the services in the building requiring additional connections to ensure continuity for emergencies.
There’s 2 ways to look at it, contact your chosen service provider to get the connection to the apartment, they see the connection isn’t there and report back to NBN for rectification, or try NBN directly and hope you get through.
Service providers should tell NBN the connection to the apartment isn’t suitable. There’s minimum standards that are required for connection.