Depends on the type of transaction.  There are various areas where default nominal codes are set, and if you are using Cost Centres and/or Departments then it could post to suspense if the code doesnt exist. 

The easiest thing to do is to try and determine where it wants to put it.  If you look in ASM > Audit List and see if it tells you on the narrative where it wants to put it.  Otherwise, if you are using Sage200c Professional, you could try turning on Current Waiting Postings, post the transaction again then check in the waiting postings as it will show the codes its intending to put it to.

Otherwise, depending on the type of transaction, you'll need to check the Nominal defaults, customer/supplier default nominal codes (set per record), etc, but as i say it will depend on the type of transaction.

Thanks for the response Gary. So, we've found that the accounts payable code doesn’t exist (2100). As this is a control account we can't set it up the way we do a normal nominal account. How do we activate the accounts payable code?

Good to hear you've found the culprit. You would still create the nominal code as per usual in the Nominal Ledger, as even though its a control account, its still a nominal code.  Once created you then go to Nominal Ledger > Utilities > Ledger Setup > Default Nominal Accounts.  Then in the Balance Sheet tab you have the Debtors and Creditors Control default selection at the top.  The way "control accounts" are set is totally different to Sage50.

Good to hear you've found the culprit. You would still create the nominal code as per usual in the Nominal Ledger, as even though its a control account, its still a nominal code.  Once created you then go to Nominal Ledger > Utilities > Ledger Setup > Default Nominal Accounts.  Then in the Balance Sheet tab you have the Debtors and Creditors Control default selection at the top.  The way "control accounts" are set is totally different to Sage50.