The mean is 9, n=400 and the standard deviation is 2

we want to know if the mean will be lower than 10 with what risk of error?

solution: 10-9=1, so 1=ta/2 * 2/20, and a=10, so there is no risk of error, we can say that the mean will be lower than 10.

I do not understand why it is possible to affirm that there is no risk of error. What 10 refers to. and how to know what is the risk of error at the end of the calcul?