Why cant i change my name too this?

So I was thinking about changing my name to: '70 Percent Water', but when I went to check this name it said it was on available. So I searched if the name was already in use and it is not. So then I checked if it was maybe to many letters or if the 70 is a problem. So I made it: '70 percentWaatee' (more letters than the original) and this was fine. What is the problem here why can't I use '70 Percent Water'?
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