This issue will occur where the contra account set to be used for journals is incorrect for various reasons and you can check which account is to be used as follows:
Click Financials then Transactions then click Journal.
Select the currency for the transaction.
Click on options
Tick Post Balance to Contra Account
Click Summary
Make a note of the account in Contra Account box
Possible reasons for the error are:
It is inactive.
It is the wrong currency.
The default is found in the currency defaults within the System area, under the currency for the transaction. The Nominal bank account is used as the default contra account and changing it to an existing one of the same currency will solve the problem.
It does not exist.
As above, check the currency default and compare the value held in the default bank account with the nominal ledger accounts. Changing it to an existing one of the same currency will solve the problem.