Translation:I want to use my phone to play games.
手机 is specifically a cell phone, not just any phone. While cell phones are usually just called "phones" in English when the distinction is unimportant or clear from context since it's shorter, this doesn't save time or space in Chinese (手机 vs. 电话), and translating 手机 as just "phone" loses the specificity of the original sentence.
I have learned that when it is implied as "my", like in, my son, my daughter, my cell phone, then it is common for a Chinese speaker to leave out the "my" when talking or writing sentences. I learned that in an earlier lesson. Because normally an adult or teenager would play games on their own cell phone and normally not grab someone else's cell phone to play games. The latter would be a less common occurrence. Sure, a child, teenager, or an adult could still not own their own cell phone or could decide to use someone else's cell phone to play video games. That is true.
I believe this is common in Chinese; the "我的"/"my" is implied, and the explicit use of this construction is much less frequent in Chinese than it is in European languages. Without further context, it seems fairly clear that it would be one's own phone that you're using for games; if it was someone else's phone, which is much less likely, that would probably be specified.
That being said, English translations that say "use the phone" or something similar should be accepted by Duolingo as these couldn't be considered incorrect either, even though that's not the preferred or most natural way that one would typically translate these sentences.
I put I want to use my cell phone to play games and I got it right, and I thought that it was incorrect because when I was talking to my pen friend piano ming (who is Chinese but came over to stay with us for a bit) told me that the way I put it which is seen in top is wrong and I need to add in another word but I didn't agree with her and I still got it right. And guess what people? I am going to China on the October month next year in 2019. My friend was like to me (by the way my pen friend piano ming) was like I shouldn't be to gassed about going to China in 2019 because being in China for a week is just like living in ordinary London but just longer but I was like to her since it would be my first time in 8 years going abroad yo China there is a reason for me to be so gassed. To all the people out their just follow your heart and if you wanna travel abroad then DO IT PEOPLE! LISTEN TO YOUR HEART