Cannot set LC_ALL to default locale: No such file or directory: en_US:UTF-8 as example

• Check if en_US:UTF-8 installed

# sudo locale-gen en_US.UTF-8

or # sudo dpkg-reconfigure locales

• Edit /etc/default/locale and make sure it contains the following lines

LANG="en_US.UTF-8"

LANGUAGE="en_US:en"

LC_ALL=en_US.UTF-8

• Logout and relogin
• Check warning still exisiting?