Hello @Sameer Munawar , Welcome to the Microsoft Q&A forum.
I could not reproduce the issue on my end, this might be a transient portal issue. To help pinpoint if this is indeed a transient portal issue. Can you try and deploy the Local Network Gateway using PowerShell?
You can run this command to create a Local Network Gateway using PowerShell
New-AzLocalNetworkGateway -Name <LNG-Name>-ResourceGroupName <RGName> -Location <Location> -GatewayIpAddress <GatewayIp> -AddressPrefix <IpAddress_prefix>
You can run this command via the Azure Portal's cloud shell
Please make sure that you are signed in into Azure before running this command.
If you get any errors while running the command do let us know. If you are successfully able to create Local Network Gateway using this command this will prove that the issue related to Azure Portal. In this case, it will be better if you could file a support case in this scenario. Please make sure to capture the browser trace as it will help debug this issue.
If you do not have a support plan, please refer to my private message below. (Please let me know if you are unable to see the private message) Thank you!