Testers could not download app from Firebase App Distribution

I finally got to resolve the problem.

The whole problem has to do with Provisioning Profiles. When you have to export for Firebase App Distribution is clearly stated in their docs that an ad-hoc export should be used, but the whole procedure and options are sparsely explained..

So here are the necessary steps:

1. In your developer account generate a new Distribution certificate.
2. Generate a new profile for distribution.
3. For the new profile select the right AppId, the Distribution certificate, choose ad-hoc, and select the Device UUIDs you want(the tester's)
4. In Xcode keep Automatic signing checked, and use your Developer profile.
5. Make sure to update your profile:Xcode/Preferences `Download Manual Profiles' button.
6. Make sure that in both Target/Building Settings/Signing and Project/Building Settings/Signing shows you iOS Developer profile under both Debugand Release not the Keychainone, but the Automatic one, else select it yourself.
7. Archive the project. Product/Archive.
8. Export the .ipa:
9. If Xcode doesn't get you to the organiser window open it Windows/Organizer
10. Select the proper App(from the left ìOS Apss`column) and archived file ( the latest is at the top of the list.
11. Choose Distribute App button.
12. Select a distribution method (Ad Hoc in this case)and click Nextbutton. Keep in mind that Each method require its own Certificate and Provisioning Profile..
13. Choose options ( I left it as is..) and click Nextbutton.
14. Re-signing the App is where things go wrong, keep Automatic ( here is where Xcode connects to your Developer account and sign the app with the Distributioncertificate and profile.
15. Once all the compiling has finished just click Export button.
16. Choose where to store the files on your computer and click Export.

Hope this step-by-step guide will be helpful to others..as the whole process is described very unclearly explained by the docs and puzzling all the info is quite frustrating..

Cheers.

Thanks guys. My issue is I built my app with export_method was app-store, the right method should have been ad-hoc. I changed my export_method to ad-hoc and it works as I expect.

If someone changed the export_method and still get the error. Please check Vincenzo's answer