How to create a VPN server with my openwrt running router I don't have public ip my isp giving me private ip from his DHCP server so how can I create vpn server withmy openwrt router
The short answer is, you can't.
The longer answer, maybe your ISP gives you at least IPv6 connectivity, in which case you could use IPv6(-only) to connect into your VPN.
The long answer, if you neither have public IPv4 nor IPv6 addresses at your disposal, you'd need some kind of external mediator for both ends to connect to. This may be a (virtual-) server somewhere in a data centre or some kind of commercial service.
Everything @slh said is correct, but I have a question -- maybe you can clarify:
Is the ISP dhcp server that you refer to coming from a modem+router combo device to which the OpenWrt router is directly connected? If so, does it have a public IP address on it's wan? And/or, can you remove this device or set it into a bridge/modem only mode?