I want to assign static ip address to VPN client endpoint

I believe a NAT Gateway is required to use a fixed IP address as described in this document.
https://repost.aws/knowledge-center/client-vpn-static-ip-address

There is currently no support for Static IP's for VPN clients. Addresses are provided from a DHCP pool configured on the VPN Endpoint. Also you will recieve a NAT address on one of the subnets you configured within your VPC

Is there a particular reason you need one?