Freezing lettuce is not recommended. Because lettuce is mostly made up of water, what little fiber there is usually deteriorates when lettuce is frozen. If it gets frozen accidentally in a too-cold refrigerator, it will usually get slimy when it thaws.
Yes, you can freeze lettuce. However, because lettuce contains a lot of water, when you thaw it out it will no be the same crispiness as it was when it was fresh. The resulting thawed lettuce will be watery and limp.