default or selected kernelversion does not have config value CONFIG_IP_NF_MATCH_STRING.
Result is shown for kernelversion 5.4.123

string match support

modulename: ipt_string.ko


Linux Kernel Configuration
└─>Networking options
└─>Network packet filtering (replaces ipchains)
└─>IP: Netfilter Configuration
└─>string match support
In linux kernel since version 2.6.14 (release Date: 2005-10-27)  
This option adds a `string' match, which allows you to look for
pattern matchings in packets.

To compile it as a module, choose M here. If unsure, say N.

source code: