According to the concept of Advaita, we say that 'Turiya', the supreme observer illuminates all the 3 forms the waker, dreamer, and deep sleep form of a human being. And also that every living being is united by Turiya. Then why is there separate karma for everyone if at the core we all are originating from a single entity?
There is a lack of understanding of Turiya. Turiya is not an observer.
Thou liest in the Cosmic Waters in the state of Turiya, which is neither absorption in the oblivion of deep sleep, nor involvement in the objective movement of the waking and dream states.
Srimad Bhagavata Purana VII.9.30
Turiya is a state beyond deep sleep, dream and waking state. It does not unite all living beings. You are really asking if all of us are Brahman according to Advaita Vedanta then why there is separate karma for everyone.
The answer is that karma only operates at the level of the physical and subtle body of everyone and not at the level of Brahman. There is different karma for every one since the physical and the subtle bodies are different for everyone.