If you are receiving a message that you have already installed the program on the maximum number of machines (two), but it is only installed on one, you need to contact Adobe Support. You can start a chat session by clicking on the blue Contact Us button under the Support menu at Adobe.com. Or you can contact Support by telephone using the numbers listed here: https://helpx.adobe.com/contact/phone.html. Tell support that you need to deactivate one of your machines.
Elements 15 cannot work with HEIC files. If you are downloading the files from an iPhone, go to the Settings on your phone and adjust the Photos app. At the bottom of the options, there are two choices for Transfer to Mac or PC. Select Automatic rather than Keep Originals. When you import the files to your computer, the HEIC files will be automatically converted to jpegs.