How do I get baby in high chair what it says to do doesn't work ?

First have an Adult Sim active and press on the infant. One of the actions should say "Pick up Infant". Have your Sim pick up the infant then press on the highchair, and action should pop saying "Put infant in highchair". If your Sim doesn't do this and a thought bubble cones out his head with either footprints or a red circle with a slash in it, that means the highchair is in the wrong position. If the part of the high chair is placed against the wall, but the side the infant sits isn't by the wall, it's in the wrong postion. For the chair to be in the right position, the back of the chair must be accessible. Hope I helped!