If you are creating a new iOS app, you will need to create a new App Identifier. Follow the following Step-By-Step post to know how.
Login to Developer.Apple.com and head to the Certificates, IDs & Profiles.
Select Identifiers from the side menu
Click the + button next to Identifiers
Select App IDs then hit Continue.
In iOS 14 you can create either an App identifier, or an App Clip Identifier. In this case, select App and click Continue.
In the Description type what this ID is about, then enter the Bundle ID you want, and whether or not you want it to be Explicit for this app, or a Wildcard that can be used by multiple apps.
Select the Capabilities you want your app to have. This is a long, and overwhelming list, but you are most likely won’t need all of them. The most popular ones are; App Groups, Associated Domains, iCloud, Push Notifications, and recently Sign In With Apple.
Then you press continue, then register, and that is it. You have created your App Identifier!
How To Submit An App To The AppStore
If you want to create a new App, you can follow my step-by-step post on how to do that here: https://kais.me/2020/09/app-submission-checklist-itunesconnect/
If you have any questions about this topic or any other related topics please leave a comment below!