Thanks for your suggestion, I have solved the installation problem I encountered when using the command Pkg.add(), although I don’t know why it works.
When I try to reinstall Julia according to the method here, I found that the last two steps of the installation part I did not do before. So I just added those two commands in the terminal
sudo cp -r julia-1.4.2 /opt/
sudo ln -s /opt/julia-1.4.2/bin/julia /usr/local/bin/julia
and then it can automatically install the packages I need.