It is not easy to prove that the hacking activity had "no effect" on the election.

"The report, reflecting the assessments of the C.I.A., the F.B.I. and the National Security Agency, stopped short of backing up Mr. Trump on his declaration that the hacking activity had no effect on the election.
'We did not make an assessment of the impact that Russian activities had on the outcome of the 2016 election,' the report concluded, saying it was beyond its responsibility to analyze American 'political processes' or public opinion."
‪Putin Led a Complex Cyberattack Scheme to Aid Trump, Report Finds http://nyti.ms/2jbRig6‬

It is not easy to prove that the hacking activity had "no effect" on the election. You need to analyze the entire political process and public opinion in detail, which is next to impossible.
One the other hand, logically speaking, it is easier to prove that the hacking activity had some effect on the election.