The system might show various error messages during the invoice confirmation cycle.

Invoice rows are not accounted - if you are the last confirmer, all invoice lines must have accounts to be approved, including lines with an amount of 0 euros.

The entry date does not belong to the allowed billing period when the rows are red - it only occurs if the date when the billing period is locked is specified in the purchase invoice settings.

The difference in invoice sums - If the difference is less than 1 euro, it is probably due to rounding and you can click the Check Amounts button (function has to be activated in purchase invoice settings first). However, if the difference is bigger, then the rows should be reviewed and corrections should be made.