The President of the United States has the legal power to tell the FBI to drop an investigation.
President George HW Bush ended an investigation of Secretary of Defense Caspar Weinberger by simply pardoning Weinberger and six others, thus foreclosing the possibility of the scandal reaching the White House. While the Special Counsel was outraged, there was nothing he could do.
Obstruction of justice does not apply, because the President is, in effect, the chief prosecutor.
At the time that Trump expressed his hope that Comey would let the Flynn case drop, Flynn was accused of nothing more than speaking with the Russians WRT sanctions. That is not illegal. As there was no reason to believe that there was any wrongdoing, it was perfectly proper to “hope” that was the end of the matter.
Before his firing, Comey swore under oath that nobody had ever asked him to drop an investigation for political reasons: https://www.youtube.com/watch?v=MLI2_MGz-E4