If you can't find the account you're looking for in the mapping dropdown options, it is likely because of one of two reasons:
The account you're looking for is not a normal account type for the field you are mapping to. For example, if you are trying to map an income to an expense account. If you want to see all accounts, simply click 'Show all accounts'.
You're trying to use a system account. These are accounts that Xero does not allow apps to use, therefore if we send data to Xero with that account it will be rejected. See below for a list of Xero system accounts. They are also marked in Xero with a small padlock image:
The following account codes are system accounts:
497, 498, 499, 610, 630, 800, 801, 820, 840, 860, 877, 960