Error installing deal.II

Hello there,
I am experiencing some issues during the installation of deal.ii on ubuntu 20.04.
I followed the steps in the manual. When I perform . / -p INSTALL_PATH, I got an error:

It seems to be a problem with my network, but I checked my network and found that I was already connected to it.
So,how should i to solve it.

It looks like your internet connection is not working correctly. You can try this yourself by running

Thanks for your answer.
I found this to be a common problem, which has been solved by reading (Installation FAQ · geodynamics/aspect Wiki · GitHub).