Hi, thank you for contacting justanswer.
So your door works fine manually, right?
If so, try this.
First, double check your battery installation, polarities of the batteries, are contacts rusted, Etc;? If so clean. Just visually check the battery compartment and the placement of the batteries. Are batteries the correct voltage? If everything looks good and batteries were installed properly and are correct, do this.
With the garage door closed, press the large button on the remote. While that is pressed, press the "learn" button, (probably red) on the back of the opener unit. The indicator light on the panel will begin to blink and the opener light will flash once. At that point, release all buttons, and try out your remote!
If this doesn't cure the problem you may need another remote?