Cant Log Into Void Launcher

I type in my username and password for minecraft or something else it just keeps saying
Invalid username/password combination
Would you like to play offline?

you will be unable to connect to any online servers

when I click yes it says
You must successfully login in once on this machine to play this mudpack offline

If you do not have a premium Minecraft account, you can buy one then it gives the link but I already have premium so what can I do to log in to play modpacks on this?