Why can't I find the error function name with GDB?

Hmmm...
...core dumps are typically debugged in the buildhost, as the executable binaries & libraries in OpenWrt firmware image are typically stripped from debug info (for flash space constraints), so gdb finds no debug materials in the binaries in the router.

Easiest would be to use the OpenWrt toolchain gdb for the proper cross-compiling target. (Note that the gdb 7.5 in your screenshot has last been used in OpenWrt in year 2013. Current OpenWrt master and 21.02 use gdb 10.1. You seem to be tinkering with ancient unsupported stuff ???)

The OpenWrt gdb is located in the build_dir.

see a recent example in:

 ./build_dir/toolchain-arm_cortex-a15+neon-vfpv4_gcc-11.2.0_musl_eabi/gdb-10.1/gdb/gdb ./staging_dir/target-arm_cortex-a15+neon-vfpv4_musl_eabi/root-ipq806x/sbin/netifd netifd.1632756907.1577.11.core

Successful debugging (finding the crashing source line) pretty much requires that you have the unstripped binary still available in the buildhost, so that you can provide that for the gdb.

Alternatively you should have unstripped versions for the binaries in the router itself.

i have selected unstripped in menuconfig, i need to clean build?

There is also something like "enabled debugging info for packages".

And yes, you need a clean build.