Does Margin do e-invoicing?

Not yet. Companies whose turnover is above ₹5 crore must get an IRN from the government e-invoice portal for invoices to other businesses, before the invoice is valid. Add your annual turnover in Settings → Company details: above ₹5 crore, every invoice in Margin shows "Your turnover means this invoice needs an IRN from the government e-invoice portal before it is valid." Margin doesn't stop you issuing it; get the IRN on the portal, or ask your CA.