FTP over internet

> Model: R8000|Nighthawk X6 AC3200 Smart WIFI Router

   Is that accurate?  Connected to what?

> I did a port forwaring from my WAN to the router on port 21, but was
> unsuccessful.

   None of that is a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

> [...] USB disk that the router X6 has.

   You're talking about using the R8000 as an FTP server?  If so, then
visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Use FTP
Within the Network" and "Set Up FTP Access Through the Internet".

> [...] it gives an error message saying that I cannot make a port
> forwaring towards the address of the router (192.168.2.1)

   You'd use explicit port forwarding that way if your FTP server were
some computer on your LAN, _not_ if it's the router itself.  When you
use the router as your FTP server, the port forwarding is implicitly
configured by the following step:

      5. Select the FTP (via Internet) check box.

> [...] I cannot access it from outside my house.

   Regarding "cannot", see "useful problem description [...]", above.
What, exactly, did you do?  What, exactly, happened when you did it?

   Does the IP address of the WAN/Internet interface of your router
match what you're using as your public IP address from the outside
world?  ADVANCED > ADVANCED Home : Internet Port : Internet IP Address

   If not, then that could be your problem.  An intermediate NAT router,
for example, could cause this.  Or an ISP using carrier-grade NAT to
conserve IPv4 addresses.

      https://en.wikipedia.org/wiki/Carrier-grade_NAT

> [...] the LAN FTP works perfectly [...]

   That's using the router's LAN IP address ("192.168.2.1"?)?  Does it
also work if you use the router's WAN/Internet IP address from a system
on your LAN?  That would suggest that the port forwarding is working.