The definition of Public Key Encryption(PKE) say that:
A PKE scheme is a triple of probabilistic polynomial time algorithm (PPT) (Gen,Enc,Dec).
The definition of PPT say:
In complexity theory, PP is the class of decision problems solvable by a probabilistic Turing machine in polynomial time, with an error probability of less than 1/2 for all instances.
Let "Find a pair key $(pk,sk)$ (public and private key)" be the problem related with the first algorithm (Gen). In the McEliece scheme, what is a example of error of any instance of the algorithm Gen? Which will be a response "No" and What is a difference between the response "No" and the error response of the algorithm Gen?