You can lie down to meditate, it is perfectly ok, it just depends on the type of meditation you are doing, and it's purpose.
Meditation lying down for sleep is a great way to prepare yourself to fall asleep, especially if you have trouble relaxing before bed.
If you are debating whether you should meditate lying down or sitting, think about what the purpose of the meditation is, and what you will be doing after.
Many people experience back pain and knee pain when first starting out with meditation, which obviously isn't very relaxing. Of course, this gets better as you keep a consistent meditation practice, but if your purpose is to relax before bed then lying down may be a better option.