Any account with a lock beside it in Xero is a system Xero account.
When mapping your Xero accounts, these locked accounts will not appear in the dropdowns as they restrict transactions or journal entries posted to them. 
If this is the case, you will need to create a new account. You can do this in Retail POS by selecting Add a new Account from the mapping dropdown menus.
These accounts will be automatically created in Xero.