The message is quite clear - you’ve made a mistake and the compiler is letting you know.
If you look at the page explaining the DateTimeToStr function you can see there is not a version which takes a string as the second argument: System.SysUtils.DateTimeToStr - RAD Studio API Documentation.
There is a function where you can do that: System.SysUtils.FormatDateTime - RAD Studio API Documentation
NOTE again that the format string in the first instance is the FIRST parameter.
You would use it like this: System.SysUtils.FormatDateTime( 'dd MMM,yyyy', aRecord.DateTime);
The function you are calling - and the second version of FormatDateTime expects a parameter of TFormatSettings.
An example of how to use TFormatSettings can be found here: TFormatSettings (Delphi) - RAD Studio Code Examples
It’s a record - you should pre-populate it first to get the operating system defaults and then override them if you need to - BUT I suggest you use the FormatDateTime function instead based on what you’re trying to do.