Ran into an error while connecting to ftp server

[RobinApplegate]

Hello! I get this error while attempting to build zyn for the first time on macos. I tried twice and failed in the same place both times. I'm on Catalina, 10.15.7, and as far as I know there is nothing abnormal about my internet connection (like blocked ports or anything like that). My firewall is turned off. Is everything alright with the server, or is there something wrong on my end?

--- Downloading.. ftp://ftp.simplesystems.org/pub/libpng/png/src/history/zlib/zlib-1.2.7.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:01:15 --:--:--     0
curl: (7) Failed to connect to ftp.simplesystems.org port 21: Operation timed out

[fundamental]

Looks like that might be a dead link. I don't recall a dependency on a specific zlib version, so finding a more current mirror likely resolves your issue e.g. https://www.zlib.net/zlib-1.2.11.tar.gz