Why can't set value of LC_ALL in locale?

OS in my local pc.

uname -aLinux debian 4.9.0-11-amd64 #1 SMP Debian 4.9.189-3 (2019-09-02) x86_64 GNU/Linux

My current locale.

localeLANG=en_US.UTF-8LANGUAGE=en_US:enLC_CTYPE="en_US.UTF-8"LC_NUMERIC="en_US.UTF-8"LC_TIME="en_US.UTF-8"LC_COLLATE="en_US.UTF-8"LC_MONETARY="en_US.UTF-8"LC_MESSAGES="en_US.UTF-8"LC_PAPER="en_US.UTF-8"LC_NAME="en_US.UTF-8"LC_ADDRESS="en_US.UTF-8"LC_TELEPHONE="en_US.UTF-8"LC_MEASUREMENT="en_US.UTF-8"LC_IDENTIFICATION="en_US.UTF-8"LC_ALL=

I want to set LC_ALL=C.UTF-8.

sudo localectl set-locale LC_ALL=C.UTF-8Failed to issue method call: Invalid Locale data.

Have a try to set LC_ALL=en_US.UTF-8.

sudo localectl set-locale  LC_ALL=en_US.UTF-8 Failed to issue method call: Invalid Locale data.

The default value for LC_ALL can only be blank(empty)?