Can a director of a liquidated company be sued for an outstanding debt?
Assuming the debt is owed to the company, then probably the answer is no. Under the usual rules of contractual privity, a third party cannot enforce a contract against one of the contracting parties. If the company has ceased to exist, its assets may have gone bona vacantia to the crown.
However, without knowing lots more about the situation, I can't be entirely sure of the answer. If the debt is sufficiently substantial to justify it, you should instruct a solicitor to advise.