My Mac version is Mojave 14.6, and if I don't upgrade to Catalina, is it possible to debug app on device of iOS 14 on xcode 11?
If not , maybe i won't upgrade my iPhone to iOS 14...
What is important is the version of XCode. If you are using XCode 11, then you can only deploy to iOS 13 or lower. Definitely don't upgrade to iOS 14 if you want to stay with XCode 11.
This is especially important if you are just learning iOS development and your resources and sample code references XCode 11. Upgrading to XCode 12 might make some of the sample code or instructions not work anymore. Best you complete your learning and then upgrade. Once you are proficient, you can upgrade and you should be able to fix any changes necessary to support the new version of XCode. Also possibly the original author will have updated their learning material as well.