Hello Bharaniraj Iyyamperumal,
Thanks for your question.
The error means that a VPN gateway might be in existence already which doesn’t support co-existence with the one that you are trying to configure. In an Azure Virtual Network, you can deploy only one VPN gateway, this is documented here.
Also please take a look at this doc "Configure ExpressRoute and site-to-site coexisting connections using PowerShell" https://learn.microsoft.com/en-us/azure/expressroute/expressroute-howto-coexist-resource-manager?tabs=new-virtual-network#add
Please let me know if you have further questions
You can mark it 'Accept Answer' if this helped.