Fix an invoice that cannot be finalized

Only owners and admins can finalize. If a check fails the invoice stays a draft, nothing is booked and no number is used, so numbering has no gap. The list under "The invoice cannot be finalized yet:" names every cause and the editor jumps to the first field concerned. The panel Mandatory content (§14 UStG) shows the same gaps before you try. Fix them, then choose Finalize and Assign number and lock again.
Your own data, under Settings, Company profile tab: the company name and address must be complete, and a Tax number or a VAT ID must be entered. Small business (section 19 UStG) must match the invoice: ticked only with the tax treatment Small business (§19), and that treatment only when ticked. A missing Register court and number only warns.
The customer: the recipient needs name, street, postal code and city, in Customer or typed in the recipient fields. A domestic invoice up to 250 EUR gross may leave out the recipient and the service date. A draft written by the agent needs its customer confirmed first (Confirm under Contacts) and not deleted.
Dates and lines: set the Invoice date and a date or period under Delivery or service (the end not before the start); "No delivery or service date" is accepted only for the small domestic case above. Every line needs a description, a quantity and a unit price, and there must be one line that is not a text line.
VAT: a 0 % line under Domestic, VAT charged is refused. Give the line its real rate or change Tax treatment to Exempt (§4) or Solar PV at 0% (§12(3)). A rate that was not in force on the service date is refused. Intra-EU supply and Service to an EU business (reverse charge abroad) need the customer's VAT ID from another EU country (see the page on invoices to the EU and abroad).
Other refusals: "The month of the invoice date is closed." means pick an invoice date in an open month or ask an owner or admin to re-open it. "The ECB rate for this currency on the service date is missing." concerns a foreign Currency: try again later or invoice in EUR. "The invoice could not be finalized. No number was used." means try again; if the answer was lost, the editor reloads and shows the invoice as issued.
E-invoice refusals (invoices to business customers): contact person, phone and e-mail under Invoice contact in the company profile; the IBAN under Bank details for invoices (only owners and admins change it); the customer's Leitweg-ID or a Reference / order number under Details; the customer's full address and Email or Leitweg-ID; a VAT ID or register number of your company (a tax number alone is not accepted); a Due on date or Payment terms. Other rules name their code and text: fix what it says. If the e-invoice check is not reachable, finalize again later; no number was used.
This client has no default categories (revenue) yet.: the client's default setup never ran, so no revenue category exists to book on. The invoice stays a draft and no number is used. Seed the chart of accounts under Settings > Accounts, then finalize again.