After you learn how to ride the skateboard, you are perhaps trying out new tricks like ollie or flips. But the problem is, instead of landing both your feet on the board, your feet land on the ground, thus not completing the trick you've started. How do you solve this problem?
There are probably 2 reasons why your feet land on the ground instead on the board: you are either scared or you are doing the trick the wrong way.
Being scared is perfectly okay. This means that you are taking skateboarding seriously and you are aware that you might get hurt if you do something stupid. But, you have to overcome this fear in order to perform the tricks. If you can't land with both of your feet on the skateboard, start with one foot.
It doesn't matter if it is your left or right foot. What matters is that you are determined to do the trick right. The key here is practice. After you can land on one foot, try landing on both feet. If the other foot still refuses to land on the skateboard, then there must be something wrong either with the execution of the trick or the form of your body. If you are not comfortable doing tricks while rolling on a concrete ground, it is okay to practice on the grass or carpet. As long as you are determined to do it right, you'll be okay.
Read articles and watch videos of the skateboarding trick you are trying to learn. You will learn quickly if you learn from the experts, or at least, people who know what they are talking about.
Now try the trick again and be determined to land both of your feet to the board. If you still can't, I suggest that you get someone to help you determine what's wrong with your execution, someone you trust and understands about skateboarding, someone who is willing to help you improve your skill. Do the trick again. Practice until you get it right. If you are still having problems with one or both feet, practice harder and don't give up.
It can be frustrating but skateboarding should be fun. So relax and keep practicing. Sooner or later you will get it right. And once you do, practice even more.