The title explains itself
I know I'm many months late, but now that the games came out, it's safe to say that the only Pokemon currently not capable of Dynamaxing are the three legendary Pokemon new to this Generation.
According to a leak on 4chan that (so far) has proven to be accurate, Dynamaxing will be available to every Pokemon, however, Gigantimaxing will only be available to certain Pokemon.
(This is an image of the leak. It correctly anticipated the Dynamax and Wild Area features before they were officially revealed, as well as multiple character names. It contains other details which are now presumed to be accurate; don't click if you don't want to be spoiled!)