@iAmSaugata 

Usually this error message: "Confliting with existing port." appears when the UPnP has opened a port, but you're trying to open the same port via Port Forwarding as well.

The router UPnP feature may interfere with the Port Forwarding if they are active at the same time.

Try disabling UPnP in NAT Forwarding menu and then make the same test to see if there's any difference.

  @terziyski 

The error I got is not from uPNP, I have uploaded a video, please have a look.

https://youtu.be/RNYBxjLR9BE

Scenario:

UDP port 51820 enabled in NAT Port Forwarding.

Mobile is connected to WireGuard VPN from Mobile Data.

Now, if I disable the NAT Port Forwarding in router, it will not blocking the port for external connection, I can still able to disconnect and connect VPN from Mobile.

Even at this point, if I reboot router, after rebooting, I am still able to connect and use same UDP port, and now if i try to enable WireGuard UDP port in NAT Port Forwarding, I am getting conflict error.

I do not think this is expected behavior from router, at least not after rebooting the router itself. It looks like, during reboot it failed to disable the port as per settings. This should be investigated, as it could lead to some issue for well-known ports.

I am not sure if this is applicable for TCP ports as well, but I will be going to check.

  @iAmSaugata 

Yes, it seems that this should be attended by TP-Link as the way it goes.