Adding an account
Accounting → New account. Give it a code, a name and a type: Asset, Liability, Equity, Revenue or Expense.
Add it under an existing account to make it a sub-account — "Fuel" under "Vehicle costs". Reports roll sub-accounts into their parent, so you get detail without a longer report.
A journal entry by hand
Accounting → Journal → new entry. Every entry needs at least two lines and the debits must equal the credits — it will not save otherwise.
Most people never need this. Invoices, bills and categorised transactions post their own entries automatically. Reach for it for depreciation, an owner's contribution, a correction, or an accountant's adjustment.
Why an account cannot always be deleted
Once something has posted to an account, deleting it would leave entries pointing at nothing. Make it inactive instead — it disappears from the pickers and stays in the history where the numbers still need it.