Why can't I buy SOMA?

[Romulator]

I'm.. Just going to lock this.

But for your information nonetheless.

• Jens is no longer at Frictional Games.
  
• SOMA is $30.00USD. It will occasionally be discounted on Steam, GOG and Humble Store at an appropriate opportunity. If your father's account balance is in the negative, then I would suggest he fixes that.
  
• SOMA will not be free to play anytime soon, that I can assure you.

If an account balance is in the negative, it typically means that account does not have any money to spend. That being said, no wonder you cannot "buy SOMA".