An assumption of CAPM is that investors do not face transaction costs. In particular, CAPM assumes that markets are liquid in the sense that you can always trade any number of shares you would like to any time. In reality, the overall market may become more illiquid in certain times and less illiquid in other times. When the market is illiquid, investors are more hindered from trading by frictions in the market. Suppose that the price of a certain security tends to go up when the overall market becomes more illiquid. Would you expect this security to have a higher or lower expected return than what the CAPM predicts?

CAPM assumes that markets are liquid and that there are no transaction costs. This means that when it gives the expected return of a security, it does so without accounting for illiquidity.

If the security is therefore affected by illiquidity such that the price goes up in such a situation, CAPM would not have accounted for this which means that the expected return of the security should be higher than what CAPM predicts to cater for this price increase.