2 things:
Material support of Assad- Iranian military commanders and troops, funding groups like Hezbollah to fight for Assad
Ballistic missile tests.
This is the long part of goals of the Iran deal. They need to start to behave as a stable country so banks and investors see them as a viable partner. Thier continued actions as a rogue state make the sanctions relief useless.
Iran is helping Asad cause they don't want ISIS showing up right on their doorstep. And if I recall there was nothing about Ballistic missiles in the Iran deal so they could do that if they wanted.
It doesn't matter though. The republicans were always gonna find something to blame Iran and cancel the deal. They have such a hard on hatred for Iran that I'm sure they rather start a war than let them get any relief.