You can't record a receipt; a message says the transaction is unbalanced
Before you can record a receipt, the amount you received must be fully allocated. That is, the total amount of the receipt (as shown in the receipt image at the top of the window) must be assigned to the accounts that are being paid. For example, if the customer is paying for a repair service, you might allocate the entire payment amount to your Repair Service Income account. Other receipts might be split between two or more allocation accounts if the customer used a single cheque to pay for multiple services that you track separately -- repair service and maintenance, for example.
When the receipt is fully allocated, the Out of Balance amount near the bottom of the window will be £0.00, and you'll be able to record the transaction.