The bank obtained and registered a judgement mortgage against your property, this lasts for 12 years. If you sell within 12 years, the proceeds will have to be paid to the bank up to the amount required to satisfy the judgement mortgage.
The bank can, within the 12 years, seek to force a sale of the property.
The six year period, and your not acknowledging the debt, refer to the statute of limitations. These are not relevant here as the bank have obtained judgement.