default or selected kernelversion does not have config value CONFIG_INET6_XFRM_MODE_TUNNEL.
Result is shown for kernelversion 6.9.9

IPv6: IPsec tunnel mode

modulename: xfrm6_mode_tunnel.ko

configname: CONFIG_INET6_XFRM_MODE_TUNNEL

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>The IPv6 protocol
└─>IPv6: IPsec tunnel mode
In linux kernel since version 2.6.18 (release Date: 2006-09-19)  
Support for IPsec tunnel mode.

If unsure, say Y.

source code:
net/ipv6/xfrm6_mode_tunnel.c
selects
CONFIG_XFRM