Cannot resolve the errors related to the Linux toolchain - GTK 3.0 development libraries

Question

Now I'm working on setting up own flutter environment on Ubuntu 20.04 machine, but cannot resolve the Linux toolchain errors.

$ flutter doctor -v

...

[✗] Linux toolchain - develop for Linux desktop
    • clang version 10.0.0-4ubuntu1
    • cmake version 3.16.3
    • ninja version 1.10.0
    • pkg-config version 0.29.2
    ✗ GTK 3.0 development libraries are required for Linux development.
      They are likely available from your distribution (e.g.: apt install libgtk-3-dev)
...

Ubuntu version:

$ cat /etc/os-release 
NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

For this in the terminal I tried:

sudo apt install libgtk-3-dev and it was already installed, so reinstalled it.

$ sudo apt install libgtk-3-dev
Reading package lists... Done
Building dependency tree       
Reading state information... Done
libgtk-3-dev is already the newest version (3.24.20-0ubuntu1.1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Checked the gtk-3 packages

$ dpkg -L libgtk-3-dev | grep '\.pc'
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-broadway-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-wayland-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-x11-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-broadway-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-unix-print-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-wayland-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-x11-3.0.pc

$ pkg-config --modversion gtk+-3.0
Package gtk+-3.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gtk+-3.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gtk+-3.0' found

$ pkg-config --libs gtk+-3.0
Package gtk+-3.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gtk+-3.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gtk+-3.0' found