My ESC's keep beeping after connecting battery

[RedSun]

I've just finished my Quad F450 build and as I tried to power it on for the first time my ESC's keep beeping and the FCU doesn't power on. It this happening cause of a faulty/wrong connection I did or do I have an issue with something from my components? My build consists of :
1. Frame F450
2. Flight Controller Unit KK2.1.5
3. Motors x4
4. ESC's x4
5. UBEC
6. Battery
7. Tx/Rx

Thanks in advance for any help and info you can provide.

 

[GlassKnees]

The only time I ran into this problem, I had something misconnected. For example, I had one of the flight control (rudder, I believe) connections reversed, so signal went to ground and vice versa. If your FCU isn't getting power, that's the first place I'd start looking - where does it get it's power? From the BEC?