As pointed out in the comment, the change from int (which is how a is created) to float (which is the result of /) is not allowed when using /=. To “fix” this the dtype of a just has to be a float from the beginning:
a=np.array([2, 4, 6], dtype=np.float64)
a/=2
print(str(a))
>>>array([1., 2., 3.])