Unsupported proxy configured: 127.0.0.1://8888. So it seems you (or somebody else) configured a bogus proxy server, it most likely should read 127.0.0.1:8888 (without the slashes (//). You need to change these values either in your apt.conf(5) (/etc/apt/apt.conf or /etc/apt/apt.conf.d/) or via the http_proxy environment variable.
How to Fix : The proxy server isn't responding - YouTube Apr 05, 2018 Proxy Server is Not Responding Error in Windows 10 Sep 18, 2015 How To Fix The proxy server is refusing connections Error
Connection Problems - Message "The configured Proxy Server
Internet Explorer Error – Proxy server is not responding Nov 24, 2014 How To Check the Proxy Server Settings on Your Computer Oct 02, 2017
Connection Problems - Message "The configured Proxy Server
Error" The configured proxy server is not responding May 19, 2020 How to solve “The configured proxy server is not