Earlier today U.S. President Donald J. Trump signed into law H.R. 3364, the "Countering America's Adversaries Through Sanctions Act."
Among other things, this law is known for enforcing additional sanctions on Russia.
In a statement to the press (available here) written after signing this law, President Trump's sentiments were expressed thus (emphasis added):
While I favor tough measures to punish and deter aggressive and destabilizing behavior by Iran, North Korea, and Russia, this legislation is significantly flawed.
In its haste to pass this legislation, the Congress included a number of clearly unconstitutional provisions. For instance, although I share the policy views of sections 253 and 257, those provisions purport to displace the President's exclusive constitutional authority to recognize foreign governments, including their territorial bounds, in conflict with the Supreme Court's recent decision in Zivotofsky v. Kerry.
Additionally, section 216 seeks to grant the Congress the ability to change the law outside the constitutionally required process. The bill prescribes a review period that precludes the President from taking certain actions...(continued)
Looking at the statement the President seems to have had multiple reservations about this law.
Consequently why did he not veto it?