Why Doesn't My Guitar Sound the Same?

Hi there!

I'm a complete beginner at guitar. I recently bought a Yamaha ERG 121C, and as far as I know it doesn't seem to have any issues. I use a mobile app to tune my guitar, and I find it works great. I've been trying to learn a song off YouTube and when I play individual notes my guitar sounds the same as the one in the video, but when I try to play the chords it just sounds off. I'm really lost as to why this is, since I'm quite sure that the tabs in the video are correct, and that I am playing the chords correctly.

Here is the video: https://www.youtube.com/watch?v=JMeoq5Qf3tQ&list=RDJMeoq5Qf3tQ&start_radio=1

Any advice or enlightenment would be greatly appreciated! Thank you!