Never used the Eagle, what I'm doing wrong here?
If you're new to eagle, then why start using it?
Eagle was bought by autodesk about 5 years ago, they changed the licensing, dropped the free version, dropped Linux support, made it much more expensive, moved it to the "cloud", and a few years later they tried to buy altium. Eagle pretty much looks like a dead end to me.
There are plenty of other programs:
https://www.eevblog.com/forum/eda/pcbeda-software-list/Your PCB shows a date in 2017, so I assume it was made in that time and by someone else. If your main consideration for using eagle is because this PCB was designed in it. Then you can also consider importing it in one of the other programs. Probably several of the programs in the list above can import eagle files. I only have experience with KiCad myself, and it's import of eagle projects is quite good, (but not perfect).