calendar icon not showing on iPhone

Check all of your home pages. Also, the Calendar app is a native iOS app and is part of the operating system. It usually appears on the home screen alone, but it could have downloaded to a folder called Extras. Search your iPhone for Calendar if you do not see it on the home screens or folders. See this to search the iPhone. Use Search on your iPhone, iPad, or iPod touch - Apple Support

If it is located in a folder, the name of the folder will appear to the right of the screen.

If you downloaded it and when you go back to the App Store it shows Open, then it is on the iPhone. What happened when you searched the iPhone as I showed you?

Not a problem. Glad you were able to work it out.