Mill claims that when Kant attempts to deduce moral duties from the Categorical Imperative, he fails to show that there is any contradiction involved in being immoral.
At best, he only shows that the consequences of everyone being immoral is something that we wouldn't want, but not something that we cannot will.
However, Mill doesn't give us an argument supporting his objection, so we don't know why he thinks the two tests don't work in the examples Kant discusses. But there are reasons to think the test is flawed.
For instance, couldn't any action be justified, as long as we phrase the maxim cleverly?