Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.But all possible actions accord with infinitely many different universal laws.
For example suppose I want to rob someone. That is compatible with the law "rob everyone in sight". It is also compatible with the universal law consisting of "never rob anyone" plus specifying one exception. That law is universal since it covers all cases (that's what universal means).
It's also compatible with, "Commit the robbery. 500 years and N seconds later, if still alive, eat a carrot. Otherwise follow whim." As N ranges from 0 to infinity, we construct infinitely many universal moralities. And we can replace the robbery with anything else we like.
The categorical imperative is, contrary to its intent, compatible with all actions, and with all moral worldviews. Another simple way: take any moral worldview you already have which advocates what you want to do, then add "or if something is not specified, follow your whim" to make it universal.
The primary flaw is that the categorical imperative incorrectly assumes that actions only accord with one universal law each.