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.

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