This message shows when you are creating a new Account but the Account number is already being used (Account numbers are unique Fields, you cannot have the same number more than once in an Area so you will need to pick a different number).
You can create a New Account by clicking on the 'New Account' icon -
or by going to Menu > File > New Account.
If you are not adding an account number then the system will automatically create a number for you. You can change what the next Account Number will start from in the Database Area settings. You can find this by going to -
Menu option, Setup > Database Areas
Click on the relevant Area
Scroll down and open the section ‘Next Reference Numbers’
Set the 'Next Account Number' value to a number you know has not yet been reached. You may need to sort the Account List and go to the end of the list to find the last number. However, be aware that the Account Number is a string field, so, for example, "40321" will appear before "431".
Click here to view a video guide covering this topic