As I understand it, the constructors cannot force you to sign unless the town hall has given the building a habitation licence. They won’t get this until there is mains water connected. They might have applied to the town hall 2 years back, but this is them trying to pass the blame. The town hall can’t give the licence until the building is finished – obviously it’s not possible for them to inspect the installations until the installations exist.
The worrying thing is that if the constructor has to re-plumb the building, cash-flow problems might well send them out of business, unless of course everybody pays them the balance owed. Not that I’d recommend doing this for a minute. You need to speak to the town hall for their side of the story, and make contact with other owners in the building to ascertain what can be done to get mains water.