Q. I'm trying to perform an in-place upgrade from 2012 R2 to 2016 but it will not let me select the option to keep apps and data, why?
A. The best practice for servers is to not perform an in-place upgrade but rather perform a fresh install of the OS and migrate workloads over however if you need to perform an in-place upgrade for some roles and applications this is supported. If when performing an in-place upgrade the option to keep applications and settings is not available it is likely you entered a product key for a lower SKU than currently used, for example if you have Datacenter 2012 R2 and enter a Standard 2016 key then the option to keep applications and data will not be available. You also cannot perform an in-place upgrade if you are changing the installation mode, e.g. moving from Server Core to Server with Desktop Experience.