Zero-determinant (ZD) strategies are a novel class of strategies in the Repeated Prisoner's Dilemma (RPD) game discovered by Press and Dyson. This strategy set enforces a linear payoff relationship between a focal player and the opponent regardless of the opponent's strategy. In the RPD game, a discount factor and observation errors are both important because they often happen in society. However, they were not considered in the original discovery of ZD strategies. In some preceding studies, each of them were considered independently. Here, we analytically study the strategies that enforce linear payoff relationships in the RPD game considering both a discount factor and observation errors. As a result, we first revealed that the payoffs of two players can be represented by the form of determinants as shown by Press and Dyson even with the two factors. Then, we searched for all possible strategies that enforce linear payoff relationships and found that both ZD strategies and unconditional strategies are the only strategy sets to satisfy the condition. Moreover, we numerically derived minimum discount rates for the one subset of the ZD strategies in which the extortion factor approaches to infinity. For the ZD strategies whose extortion factor is finite, we numerically derived the minimum extortion factors above which such strategies exist. These results contribute to a deep understanding of ZD strategies in society.