Kernel - make menuconfig error

Working with a Headless Raspberry PI 4 with an Ubuntu Server 18.04.4 LTS.. I am trying to configure the source kernel. I have installed libncurses-dev, kernel-package, qt4-dev-tools.

When I run 'make menuconfig' in the directory /linux-5.7, it shows:

  LEX     scripts/kconfig/lexer.lex.c
/bin/sh: 1: flex: not found
scripts/Makefile.host:9: recipe for target 'scripts/kconfig/lexer.lex.c' failed
make[1]: *** [scripts/kconfig/lexer.lex.c] Error 127
Makefile:588: recipe for target 'menuconfig' failed
make: *** [menuconfig] Error 2

Lost on what to do, everything I have read hasn't helped so far. My goal is to start programming device drivers, and to do so I need a configured and built kernel source tree on disk.

Solution 1:

In the Ubuntu's Build Your Own Kernel guide it is stated that you have to install following programs :

sudo apt-get install libncurses-dev flex bison openssl libssl-dev dkms libelf-dev libudev-dev libpci-dev libiberty-dev autoconf

to be able to compile the kernel from source successfully.

Solution 2:

flex is not installed. Install it by

sudo apt install flex