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