Using an imac as second screen not working...

Hello,

I own a Late 2013 Imac 34gh with full ram on el captain, I own a 13" 2011 early macbook pro.

I have high sierra on the macbook pro, so to see if working on a web devoppement would work better on my laptop since it has a most recent OS I searched the net to use the Imac as a second screen.

I found lots of video showing that just the lightning cable is now doing the job, so I bought one on amazon but cheaper than the apple one.

It's just a cable, so I tried and when pressing the comand key +F2 nothing happens.

Was wondering if my Imac would have a different configuration or it's el captain that stops it from working? Seeing the specs my comps should be working together!

Any advice? thank you