Seeing the same issue here as well. Filed report in Feedback Assistant, recommend everyone do the same. My app installation error in TestFlight says, "profile can't be installed". FB13975325 (TestFlight won't download apps)

I was having issues getting manual signing to work, so ultimately that didn't do it for me. I got it working unscientifically, but ultimately I think these things helped: in Keychain Access I deleted all my expired certificates like the person above me said: Xcode > Preferences... > Accounts > Manage Certificates... > + > Apple Distribution. after having it set to manual signing and it not working I turned manual signing back on. It told me I didn't have a team selected (I did) so I selected "None" for team and then selected my team again as always: delete derived data, clean build