failed to get EHDR from /sys/kernel/btf/vmlinux when compiling examples

Hello! First of all, by having a look at README, this tool looks so cool!

I am following the instructions to compile the examples under `examples/src`.
I generated `libBPFCov.so`. My bpftool version is v5.4.162. When running `make` I get:
```
make: command: Command not found
MKDIR vmlinux/
VMLINUX vmlinux/vmlinux.h
libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux
Error: failed to load BTF from /sys/kernel/btf/vmlinux: Unknown error -4001
make: *** [Makefile:77: /home/ubuntu/dev/bpfcov/examples/src/.output/vmlinux/vmlinux.h] Error 95
make: *** Deleting file '/home/ubuntu/dev/bpfcov/examples/src/.output/vmlinux/vmlinux.h'
```

Apart from the `Command not found` error, looks like I cannot get EHDR from the vm "embedded" btf.
My machine is:
`Linux 5.11.0-1025-aws #27~20.04.1-Ubuntu SMP Fri Jan 7 13:09:56 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux`

Also, I tried to use bpftool with version 5.11.22, however it doesn't have the "skeleton" feature:
```
bpftool version -j
{"version":"5.11.22","features":{"libbfd":false,"skeletons":false}}
```
Might be that v5.4.162, does not support skeleton as well. However I do not get such an indication from Makefile.
My gut feeling is that the issue is with bpftool version. However, version 5.11.22 looks like the most updated for my OS, and if this one does not work, do you know which version might support skeleton feature? Or maybe, the problem might not be related to bpftool version.
Thank you!