If I may get a word in, we don't recommend using the Jar launcher as we only target the modern, native launcher.
"Missing Minecraft Resource" is a rather common issue for Twitch users and I usually recommend using Twitch only to install and manage packs, not to actually handle launching them.
Yes, this means managing the base Minecraft and Forge versions manually, but it also means Twitch isn't stepping on Forge's toes and that you can launch Twitch modpacks with just the regular launcher. (video in the Logs -> Twitch Launcher section of my signature below)