We commonly say adaptation. I personally have never used adaption, nor have I heard or read people use it.

They are synonyms, both meaning "the act of adapting." However, you will almost never hear "adaption" in speech, perhaps see it once or twice in your life in writing, but 99% of the time it is replaced with the word "adaptation." I am a native speaker and had to check to make sure "adaption" was actually a real word, if that gives you a hint at how rarely it is used, lol! :)