Please be informed that this is not an error message, it is a warning message to imply that your current (today's) date is more than 18 period. Our system can only key in up to 18 period, you may refer below example to understand better.
Scenario:
Financial Year: 1/1/2022 - 31/12/2022
Today's Date: 27/12/2023
From here, period 18 is only up to 30/6/2023. Hence when you click Invoice or any related, the warning message will prompt out to mention that you can only key in transactions from period 1 to 18 and cannot key in any transaction for today's date.
Note: Proceed with the Year End Processing to key in any transactions.