What problem are you having signing in?
Your username should be the one that you originally signed up with, it is SEPERATE from your Roblox account and will not change.
If you haven't signed up yet, then you can do so here.
If you are still stuck please raise a ticket in our discord server - discord.gg/metaverse.
Sorry to hear that! We're in beta right now, and it looks like you've found a bug.
Let us know what the problem is - email your Roblox username and a message to Support, or post a message in #help on our Discord.