Turkey is a U.S. treaty ally through its NATO membership. Since Congress has ratified the NATO treaty, it means that U.S. law binds America to protect Turkey, because of which we have supplied Turkey with American military goods. The problem is that Turkey has been increasingly aligning itself with Russia. Which is not only not a NATO signatory, but is the entity which NATO was created to protect Europe from.
For instance, Turkey recently purchased the Russian S-400 missile defense weapon system. This move angered the United States because Turkey’s military has typically relied on American technology and secrets. Integrating Russian weapons could expose American military secrets to Russia. It would have been good for America if, for instance, the greatest dealmaker in history had gotten Erdogan to cancel its S-400 purchase in return of greenlighting their operations against the Kurds in northern Syria.
It would have been a bad deal. But at least America would have gotten something in return for sacrificing its interests.
So far as we know, he got nothing of the sort. Or, more accurately: He got nothing.