Cheers, developer of Fabric here. adds forum account to big stash of forum accounts
The Twitch launcher does not, at this time, support Fabric - it only supports modpacks based on Forge. MultiMC is a bit more complex (and versatile!), but the Fabric wiki should contain a guide on creating a Fabric instance. You might be able to find better assistance on the Fabric community spaces, say the Discord or the IRC.