Consulted documentation for Kali 2020.3 on how to configure apt to utilize an HTTP proxy. Consulted documentation on:

https://kali.training/topic/advanced-apt-configuration-and-usage/

specifically, the section that specifies how to add an HTTP or FTP proxy:

There are many other helpful configuration options and we certainly can’t cover them all, but one we will touch on involves network connectivity. For example, if you can only access the web through a proxy, add a line like Acquire::http::proxy "http://yourproxy:3128". For an FTP proxy, use Acquire::ftp::proxy "ftp://yourproxy".

The instructions don't mention it, but in order for those configuration options to be valid, they need to be terminated with a semi-colon (;). The paragraph should read:

There are many other helpful configuration options and we certainly can’t cover them all, but one we will touch on involves network connectivity. For example, if you can only access the web through a proxy, add a line like Acquire::http::proxy "http://yourproxy:3128";. For an FTP proxy, use Acquire::ftp::proxy "ftp://yourproxy";.