FBI refutes Trump claim that Clinton’s private email server was hacked by China

how do you prove a negative?, all you can say is that within the bounds of the technology and skills they have to hand, they where unable to detect any intrusion, that does not mean that there is a guarantee that there was none.

The risk that there was may be very low, but its still there.

The only secure system is an air-gaped one, and even then there are evolving threats that circumvent that protection.