Why is there no A/R account when entering invoices?

If you are on a cash-basis accounting system, you will not have an A/R account. In this type of system, the software tracks items and services sold (through invoices). However, the general ledger is not updated until payment is received and a deposit is recorded. When a deposit is recorded, cash is debited and the income account is credited.

When using an accrual-basis accounting system, the A/R account is used to offset the income account which is recorded in the general ledger when the invoice is entered. At that point, A/R is debited and the income account is credited. Later, when payment is received, cash is debited and A/R is credited.