Invalid Data. External IP Address!

> My model is CG3700EMR [...]

   I don't see that model at http://netgear.com/support .  Also, a
firmware version can be helpful as well as the (actual) model number.

> [...] when I try to add an external IP Adress it just comes up with
> the error "Invalid Data. External IP Adress!"

   Where are you trying to "add an external IP Ad[d]ress"?

> NetgearError.png

   I see the error message, but not what you put in to cause it.  If
your Web form looks like mine (D7000), then the "Server IP Address" is
the LAN address of the local server, not any external IP address.
(Which is why that field is pre-populated with the right stuff for all
but the rightmost octet.)

> Not with the IP's from IP chicken either.

   There's more than one external IP address?

   What's the LAN IP address of this "Minecraft server" system?  That's
the target in the port-forwarding rule.  You don't need to specify the
external IP address.  The router already knows its own WAN/Internet
address (probably better than you).