We are porting an Android app to iOS for a client. They have added our account to their iTunes Connect account and created an app entry. We developed the app using the bundle ID
None of the above answers worked for me, and as said in the original question I had also to keep the same bundle identifier since the app was already published in the store by the client.
The solution for me was to ask the client to change my access from App Manager to Admin, so that I had "Access to Certificates, Identifiers & Profiles.", you can check if it is the case in the App Store Connect => Users and Access => and then click on your profile (be sure to choose the right team if you belong to multiple).
Once you are admin go back to Xcode and in the signing tab select 'Automatically manage signing', then in Team dropdown you should be able to select the right team and the signature will work.