Let us say there is a Turing Machine $M$ and a string $x$. So now I want to know if $M$ accepts or rejects the string or loops on it. So, I feed the string to $M$. So, if $M$ is accept or reject the string it would halt after a finite time and I would get my answer. But how do I know if the machine loops on the string or not.
One possible way is to see if a configuration repeats itself. However, the machine has no way of knowing if the configuration has occurred before because it cannot store them . So, we would never get an answer. So, how will I be able to tell whether or not $x$ is accpted or rejected or looped upon by $M$.