The quotes prevent the expansion of ~.
Use:
CLIENT_BUILD_DIR=~/Desktop/TempDir/if [ ! -d "$CLIENT_BUILD_DIR" ]then mkdir "$CLIENT_BUILD_DIR"fi
The ~ character is not reinterpret when used in a variable.
~
You can use CLIENT_BUILD_DIR="$HOME/Desktop/TempDir/" instead.
CLIENT_BUILD_DIR="$HOME/Desktop/TempDir/"
mkdir ${CLIENT_BUILD_DIR} will do. No directory will be created if it already exists.
mkdir ${CLIENT_BUILD_DIR}