It says my MMR cannot be found, even though I have played more than 100 ranked games.

MMR indicates your skills and abilities. You will get better as time elapses, but what we want to know is your current skills. Thus, this MMR is based on the recent data, not the past ones. In other words, it does not matter whether you have played 10 matches or 300 matches so far. You must have many recent games to check your MMR.

You will get the most accurate data if you try again after playing 5 to 10 solo-queue ranked games.