Visual MODFLOW Flex, prefers a regional setting. If you are in a country and your computer is not set to regional English and you're getting the below error, these steps will help you resolve the issue.
Please be advised that Visual MODFLOW Flex has been designed based on English system. You may try modifying your Region and Language Settings by: selecting "English (United States)" for Format and "United States" for Location; also, as indicated by the message, making sure the symbols are set as the required characters in "Additional settings". Then it should work. Please refer to the screen captures below.
You can go to Control Panel>Clock, Language, and Region> Change, date, time or number formats> Region and Language setting > Additional Settings.. > and make sure of the Decimal symbol, Digit group symbol and the Separator Symbol to be the same as your error message in Flex: