This is stated in the notification in the US Federal Register.
According to the document, the head of the SSB, Ismail Demir, and three other officials who work in it was sanctioned. They will no longer be able to visit the United States, their assets in American jurisdiction will be blocked and Americans will be banned from doing business with them.
The sanctions are imposed under Countering America’s Adversaries through the Sanctions Act (CAATSA).
What kind of agreement was concluded in 2017, between Turkey and Russia is not specified, but it can be assumed that we are talking about a contract for the supply of Moscow to Ankara S-400 anti-aircraft missile systems.
Earlier, Turkey said that it would not abandon the S-400 anti-aircraft missile systems purchased from Russia, even despite the threat of US sanctions.
Recall that NATO explained why Turkey does not need Russian missile systems — they are incompatible with the technologies of the Alliance and can not be integrated into its air defense.