It is possible to knock out an alpha rex, but it is not easy. There are a few things that you need to take into account when trying to do this. First, the alpha rex has a lot of health and will take quite a bit of damage before going down. Second, the alpha rex is very fast and agile, making it difficult to hit. Lastly, the alpha rex has a powerful bite that can cause serious damage.
If you are able to find its weak spot, however, it is possible to knock out an alpha rex with one hit. The head is the most vulnerable area on the alpha rex, so if you can land a blow there it will go down much easier. It is also important to remember that the alpha rex can only be knocked out, not killed outright. Once it wakes up it will be just as strong as before and will likely be even more angry.