I bought my house in 2012, sold it in 2021. It won't let me leave the ACB blank and I don't think it pertains to my situation as the help only talks about acquiring.

Unfortunately, you can not use the "0" amount for the adjusted cost base when you try to claim the capital gains or losses from selling your own property (house) in your 2021 income tax return. 

The adjusted cost base (ACB) is usually the cost of a property plus any expenses to acquire it, such as commissions and legal fees. The ACB also includes capital expenditures, such as the cost of additions and improvements to the property. You cannot add current expenses, such as maintenance and repair costs, to the cost base of a property. That's why when you try to enter 0, the system will not accept it.  However, there are some special rules that allow you to consider the cost of the capital property to be an amount other than its actual cost. 

We would suggest you use this link from the Canada Revenue Agency (CRA) website to give you more information about the Adjusted cost base (ACB).

For calculating ACB, you can use this link from our website about Calculating Adjusted Cost Base. 

Thank you for choosing TurboTax.