First, let’s take a look at the warning information

  WARNING: The script normalizer is installed in '/home/llxy/.local/bin' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.

Solution: Configure environment variables

gedit ~/.bashrc
# Add the path at the end of the file, which comes from the warning issued when installing pip in the first step
export PATH=/home/llxy/.local/bin:$PATH
source ~/.bashrc