When first starting Peresoft Cashbook you must be able to perform a 'take-on' that is done via a take-on batch in which you enter the brought forward balance, and all un-reconciled transactions.
We suggest that this take-on is done in the prior month, i.e. if you want to create your Cashbook for May, enter this take on batch in April.
See Setting Up the Cashbook for full instructions on how to take on your Cashbook.