Big mistake on your part there. Taking time off before the apartment was completed.
You can only close and withhold money by agreement. If builder does not agree to you withholding money, he won't close. If he does agree, you can withhold money. You should be quite clear upon what basis you are withholding and a time frame after which you keep that money and complete the works yourself.
What can you do? Wait for it to be finished or threaten to walk away. if you do walk away, you may not get your deposit back.
Best to ask the solicitor for his/her best advice as to how to deal with the issue.
mf