To upload builds, update store texts, and offer bundle identifiers or package names for selection, the platform has to talk to App Store Connect and the Google Play Console. For that it needs credentials meant for machine access, not your personal login. Without these credentials, the platform cannot fully generate the store-specific build and release configuration of a project and only produces debug builds for internal testers.
Where the credentials live
Store accounts belong to the organization, not to a single project. You create them in the sidebar under Store accounts via Add access; afterwards you can select them when creating or editing any app of the organization. When you create an entry, the platform verifies the credentials against the respective service before they can be saved, and then makes them available to the generated projects and pipelines. Use dedicated technical accounts with the minimum permissions required rather than the credentials of a person.
The two accounts
For Apple you store an App Store account with an API key from App Store Connect plus the team ID and team name; it covers iOS and macOS. It requires a membership in the Apple Developer Program. Where the values come from is described under App Store credentials.
For Google you store a Play Store account with the JSON key of a Google Cloud service account that you have granted access in the Play Console. It requires a developer account in the Google Play Console. The steps are described under Play Store credentials.
Credentials for Windows signing and the Microsoft Store belong to desktop applications.