If you are trying to build the kernel on FreeBSD and you see an error message that says "don't know how to make buildkernel. Stop," it means that the make command was unable to find a build rule for the "buildkernel" target in the Makefile.
There are a few potential causes for this error:
You may be in the wrong directory. Make sure you are in the root directory of the FreeBSD source tree, and not in a subdirectory.
You may not have the necessary tools installed. To build the kernel, you will need the GNU Compiler Collection (GCC) and the GNU Binutils. Make sure these tools are installed and in your PATH.
You may be using an older version of FreeBSD. The buildkernel target was removed in FreeBSD 11.0 and later. If you are using an older version of FreeBSD, you can try using the "build" target instead.
There may be a problem with the Makefile. If none of the above solutions solve the problem, there may be an issue with the Makefile itself. You can try running "make -d" to see a verbose output of the build process, which may give you more information about what is causing the error.
If you are still having trouble, you may want to try asking for help on the FreeBSD mailing lists or forums. There are many experienced FreeBSD users who may be able to help you troubleshoot the issue.