There's a paper trail to prove he *was* briefed--probably multiple times:

[demmiblue]

7. Trump Says He Was Not Briefed On Russian Bounties Because Intelligence 'Not Credible'

In a tweet late Sunday night, President Trump said the intelligence community told him he was not briefed about allegations Russia had offered the Taliban bounty payments to kill Western forces — including U.S. troops — because it did not find the reports credible.

Intel just reported to me that they did not find this info credible, and therefore did not report it to me or @VP. Possibly another fabricated Russia Hoax, maybe by the Fake News @nytimesbooks, wanting to make Republicans look bad!!! https://t.co/cowOmP7T1S
— Donald J. Trump (@realDonaldTrump) June 29, 2020

"Intel just reported to me that they did not find this info credible, and therefore did not report it to me or @VP. Possibly another fabricated Russia Hoax, maybe by the Fake News @nytimesbooks, wanting to make Republicans look bad!!!", the president tweeted.

The tweet reaffirmed an earlier denial by the president of a New York Times report that intelligence officials had briefed him on the allegations of malign Russian activity in Afghanistan.

The Washington Post reported Sunday night that the bounties are "believed to have resulted in the deaths of several U.S. service members," a conclusion based on interviews with captured militants.

https://www.npr.org/2020/06/28/884407572/trump-denies-briefing-on-russian-bounties-reportedly-placed-on-u-s-troops