I completed the mission from fyber board king to level 42 but I didn't receive any reward from them, I've chatted with Dragon City support until now and haven't received a reply yet and now I feel sad that I've wasted time getting nothing from my hard work, is there any way I can make it work? can i complain to fyber for my hard work? I can't find their email or their website

You most likely need to contact them. If you look at the bottom of the offer, then you will see: Not earning Gem?

Look at the offer before complaining to the Socialpoint forums..
And before you ask, no, I am not a DC dev or mod. I just want to try and help.

See here on how to contact the 3rd party company.

I had this issue with Fyber a number of times. I was never issued the missing gems. They ask for screenshots to prove you completed the task, but why would you take screenshots? You trust the software to do its job.

However, SP have changed the supplier recently, haven't they? Shouldn't be Fyber now.