Hello @Matthew Campbell , welcome to the Microsoft Q&A forum.
I tried setting up a similar NAT rule at my end, and I did not face the issue above and the rule was saved correctly.
The issue here might be due to a transient portal error. Can you try below mentioned steps and see if that helps in resolving the issue?
- Can you try clearing cache and cookies for your browser and then try adding the NAT Rule? if possible using a different browser can help as well. Please let me know if the issue still persists.
- Can you try enabling the NAT rule using PowerShell commands? This might help us to pinpoint the actual issue. You can run the below mentioned command to add a NAT rule. You can run this command using Azure Cloud shell. $Gw = Get-AzVirtualNetworkGateway -Name "<your-Gw-name>" -ResourceGroupName "<RG_Name>"
$Egress = New-AzVirtualNetworkGatewayNatRule -name "<rule_name>" -type "static" -mode "EgressSnat" -InternalMapping @("<IPaddress>") -externalmapping @("IPaddress") -InternalPortRange "<port>" -ExternalPortRange "<port>"
Set-AzVirtualNetworkGateway -VirtualNetworkGateway $gw -NatRule $Egress
Hope this helps! Please let me know if you have any additional concerns. Thank you!