Can not connect to http://www.swi-prolog.org/pack/query

Does this help?

Package install errors - How to resolve after SWI-Prolog changed to using HTTPS


EDIT

On my system this is the response.

Welcome to SWI-Prolog (threaded, 64 bits, version 8.5.10)
...

?- pack_install(prologmud).
% Contacting server at https://www.swi-prolog.org/pack/query ... ok
Install prologmud@2.0.3 from GIT at https://github.com/TeamSPoon/prologmud.git Y/n