~
Seems like an outdated version.
Make sure to update your package manager and other packages.
root$ sudo apt update && sudo apt upgrade
Then install latest protobuf using
sudo apt install protobuf-compiler. On current Ubuntu it should be 3.12 or 3.14, meanwhile you've somehow got 3.6
root$ protoc --version
Command 'protoc' not found, but can be installed with:
snap install protobuf # version 3.14.0, or
apt install protobuf-compiler # version 3.12.4-1ubuntu7
See 'snap info protobuf' for additional versions.
root$ sudo apt install protobuf-compiler
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following additional packages will be installed:
fontconfig-config fonts-dejavu-core libc-dev-bin libc-devtools libc6-dev libcrypt-dev libdeflate0 libfontconfig1 libgd3 libjbig0 libjpeg-turbo8 libjpeg8 libnsl-dev
libprotobuf-dev libprotobuf-lite23 libprotobuf23 libprotoc23 libtiff5 libtirpc-dev libwebp7 libxpm4 linux-libc-dev manpages-dev rpcsvc-proto zlib1g zlib1g-dev
Suggested packages:
glibc-doc libgd-tools protobuf-mode-el
The following NEW packages will be installed:
fontconfig-config fonts-dejavu-core libc-dev-bin libc-devtools libc6-dev libcrypt-dev libdeflate0 libfontconfig1 libgd3 libjbig0 libjpeg-turbo8 libjpeg8 libnsl-dev
libprotobuf-dev libprotobuf-lite23 libprotobuf23 libprotoc23 libtiff5 libtirpc-dev libwebp7 libxpm4 linux-libc-dev manpages-dev protobuf-compiler rpcsvc-proto
zlib1g-dev
The following packages will be upgraded:
zlib1g
1 upgraded, 26 newly installed, 0 to remove and 2 not upgraded.
Need to get 3888 kB/11.6 MB of archives.
After this operation, 50.4 MB of additional disk space will be used.
Do you want to continue? [Y/n]
[...]
root$ protoc --version
libprotoc 3.12.4
The following is also helpful to see what is installed and where.
root$ sudo apt install plocate
[...]
root$ locate libprotobuf
/snap/lxd/22923/lib/x86_64-linux-gnu/libprotobuf-c.so.1
/snap/lxd/22923/lib/x86_64-linux-gnu/libprotobuf-c.so.1.0.0
/usr/lib/x86_64-linux-gnu/libprotobuf-lite.a
/usr/lib/x86_64-linux-gnu/libprotobuf-lite.so
/usr/lib/x86_64-linux-gnu/libprotobuf-lite.so.23
/usr/lib/x86_64-linux-gnu/libprotobuf-lite.so.23.0.4
/usr/lib/x86_64-linux-gnu/libprotobuf.a
/usr/lib/x86_64-linux-gnu/libprotobuf.so
/usr/lib/x86_64-linux-gnu/libprotobuf.so.23
/usr/lib/x86_64-linux-gnu/libprotobuf.so.23.0.4
/usr/share/doc/libprotobuf-dev
/usr/share/doc/libprotobuf-lite23
/usr/share/doc/libprotobuf23
/usr/share/doc/libprotobuf-lite23/changelog.Debian.gz
/usr/share/doc/libprotobuf-lite23/copyright
/usr/share/doc/libprotobuf23/changelog.Debian.gz
/usr/share/doc/libprotobuf23/copyright
/var/cache/apt/archives/libprotobuf-dev_3.12.4-1ubuntu7_amd64.deb
/var/cache/apt/archives/libprotobuf-lite23_3.12.4-1ubuntu7_amd64.deb
/var/cache/apt/archives/libprotobuf23_3.12.4-1ubuntu7_amd64.deb
/var/lib/dpkg/info/libprotobuf-dev:amd64.list
/var/lib/dpkg/info/libprotobuf-dev:amd64.md5sums
/var/lib/dpkg/info/libprotobuf-lite23:amd64.list
/var/lib/dpkg/info/libprotobuf-lite23:amd64.md5sums
/var/lib/dpkg/info/libprotobuf-lite23:amd64.shlibs
/var/lib/dpkg/info/libprotobuf-lite23:amd64.triggers
/var/lib/dpkg/info/libprotobuf23:amd64.list
/var/lib/dpkg/info/libprotobuf23:amd64.md5sums
/var/lib/dpkg/info/libprotobuf23:amd64.shlibs
/var/lib/dpkg/info/libprotobuf23:amd64.triggers