Why could the Sanhedrin not say anything against the apostles' miracle?
The healed man was standing with the apostles in complete health and strength, so the miracle was visible and undeniable. The council acknowledged that a notable miracle had been done and was manifest to all Jerusalem, and they could not deny it. Therefore they could say nothing against it.
Les explains that the council’s inability was not because the apostles had impressive credentials; they were perceived as ignorant, unlearned, and uneducated. What the council marveled at was that these men had been with Jesus, and that three years with Him was better than years in a seminary or the University of Jerusalem. Yet the decisive evidence was the healed man standing there in complete health and strength. Because the notable miracle was manifest to all who dwelt in Jerusalem and could not be denied, they could say nothing against it. They commanded the man to go aside, conferred among themselves, admitted they were up against it, and then threatened the apostles not to speak or teach in the name of Jesus. Les emphasizes that their prohibition focused on the Name, not on the message that Christ died for the sins of the world, was buried, and rose from the dead.
And beholding the man which was healed standing with them, they could say nothing against it.