It all depends on the error you got.
minecraft does have auth servers they run to make sure your acct is legit. Once and awhile they have issues and can stop anyone from logging in. the other issue that can possibly happen are:
wrong version, not enough ram. and wrong / outdated java version.
Also a system that doesnt have at least 4-6gb of ram to use for the game
java =
java 8 64 bit latest version. you can only have one installed so the default path uses 64 bit only
(it just makes life easier) https://java.com/en/download/manual.jsp
Technic Launcher-
our pack uses the technic launcher witch needs about 4gb ram assigned to it https://www.technicpack.net/download
Xi Minecraft Server Pack-
our pack we use is the1.7.10 pack
https://www.technicpack.net/modpack/the-1710-pack.453902
Minecraft Auth Server Help-
to check minecraft auth server availability
https://help.mojang.com/
so in summary a 64 bit operating system with 4-6gb of ram is recommended and bare minimum
the latest java 64 bit has to be installed
technic launcher with 4gb assigned in the options menu
1.7.10 pack added to launcher ver 97 was the last update about 10 months ago
and the server ip added to the muti tab once the game boots up (this can take about 3 min to happen just be patient)
mod-mc.xtremeidiots.com
with all that in place you should be good to go and log in should be fine.
We do not run a whitelist is open to the public
if any questions come up let us know.