The King technically has the power to dissolve Parliament, but it would not actually happen.
As explained on
parliament.uk, the Dissolution and Calling of Parliament Act 2022 “revived the power of the monarch to dissolve Parliament, at the request of the prime minister of the day” when it replaced the Fixed-term Parliaments Act.
Given that the monarch exercises this power at the request of the prime minister, this essentially means the prime minister can choose when to call a general election. The King would not do so unless specifically requested. In such a scenario, Ms Truss would likely have lost a confidence vote.