We’ve had this issue as well. The solution is to be sure to use the correct developer id - this is sometimes different than the one that xcode seems to display - I’m not sure why.
A reliable way to obtain the correct developer id is the following. Open your project in xcode and change the signing settings to the one that you want to use to sign your app:
Build once to ensure that signing works as expected. Now close Xcode and open your project’s project.pbxproj in a text editor. project.pbxproj is inside your projects Xcode project package (so for the JUCE demo it’s located at JUCE/examples/Demo/Builds/iOS/JuceDemo.xcodeproj/project.pbxproj). Then search for DEVELOPMENT_TEAM =. This will be your developer id - make a note of it. For some reason this id does not match the one that Xcode shows.
You can now use the above id in the projucer and JUCE will correctly generate your xcode projects for signing with your developer id.