I am also having this issue with Xcode 13.4.1 trying to import the Mapbox package (instructions here: https://docs.mapbox.com/ios/maps/guides/install/ ) from their Github repo. I have deleted and reinstalled Xcode, but it still hangs at "Preparing to validate...".

I had this happen to me today. I went into Keychain Access and noticed I had an expired personal developer certificate that was being referenced by our distribution certificate. I also had multiple personal developer certificates that were not expired. All the distribution certificates were showing as "Untrusted" in Keychain. I deleted the expired personal certificate, then all the distribution certificates came up as "Trusted." I then re-opened Xcode and successfully built an Archive of my current project for distribution on the App Store.