Cant update my iphone 6 plus to ios13

Just to expand on stedman1's reply..

here is a list of devices that will run iOS 13 in case you are thinking about upgrading your phone

Supported iPhone models - Apple Support

here is a table that shows the latest release for your phone being 12.4.7

Apple security updates - Apple Support

Correct, as 12.4.7 is the highest possible for that model.