I had a similar issue after upgrading to 21.04. Package linux-headers-<version> silently failed to build these config files due to lack of packages flex and bison.
sudo apt install flex bison sudo dpkg-reconfigure linux-headers-<version>
solved the problem, although the build still fails, but in a later stage.
I had a similar issue after upgrading to 21.04. <version> silently failed to build these config files due to lack of packages flex and bison.
Package linux-headers-
sudo apt install flex bison <version>
sudo dpkg-reconfigure linux-headers-
solved the problem, although the build still fails, but in a later stage.