I cant drag and drop items either on activity_main.xml
file or content_main.xml
file. From my research, I learned that I have to downgrade from android API 24 to android API 23, but when I click on API 24 image to downgrade I can't find API 23.
Check the image and download API 23, then restart Android Studio and you will see API 23 too there.
You will see this window, then start the download: