I am very new to music theory. From the website http://www.myvocalrange.com, it states that the vocal range for the song is from is Eb4-Ab5. But when I used the app on my android phone to measure the pitch while playing the song on YouTube to the speakers, it seems like the highest pitch is only Ab4, not Ab5... What am I missing here? Thanks in advance for the help.
Your website is wrong. This is exactly why I try never to rely on such applications; they tend to be wildly inaccurate. Of course, you could perform the song in the range suggested by the website, which might be easier if you're a female with a high voice, or a male with an extremely high voice as compared to averages. I won't tell you what the real range is, but in the future, my advice is to always verify any information about a song you receive online with something more reliable, preferably yourself, since technology for that kind of thing is known for being off by octaves and the like. If you want someone to tell you the range, ask elsewhere; such requests are off-topic on this site.
As a reality check when determining octaves, you should ask yourself whether it makes sense for Ed Sheeran to be singing an A♭5 in chest voice. That would be absurd, and certainly outside of his capabilities. A♭4 is much more reasonable, but that doesn't mean your app is correct, it just means it's likely the website's wrong. If it were a female singing the song, you could ask yourself whether it makes sense for her to be singing an E♭3. That's pretty low, but some women can do it, so it's less clear. E♭4 as the low note would be okay too. Also, the timbre of the note should help you determine the octave. With practice, it's easy to tell that a male singer is singing C5 versus C4 as long as they're in the same register both times.