Your business name is the name at the top of every quote and invoice you send. It's also the name your customer sees an email, text or WhatsApp message arrive from.
Where to set it
Settings → Workspaces → Business, in the Business name box. Type it in and save.
It's also the first card on your Quick Start list, and you can type it straight into the card without going anywhere.
You can't send a quote or an invoice until it's in
Until a real business name is saved, Mucka won't let a quote or an invoice go out. That covers emailing it, sending it on WhatsApp, and copying the share link.
Everything else still works. You can build quotes and invoices, edit them, look at the preview and save the PDF. It's only the moment the document would become your customer's that waits on the name.
The reason is simple: an invoice has to say who's asking for the money.
Why it might say something odd already
When you signed up we put a stand-in name on your account, usually your own name with "'s Business" after it, so the box wasn't empty. That stand-in doesn't count as your business name, and it's the thing this is asking you to replace.
If you trade under your own name, that's a perfectly good answer. Just type it in.
Changing it later
Change it any time in Settings → Workspaces → Business. It's read fresh every time a document is drawn, so changing it updates what's printed on quotes and invoices you've already raised.
Mucka will never change your business name for you once you've set one. If you mention a different name during a chat, it's kept as you saved it.