@Soukbench check you keyChain and see if your distribution certificate's private key is marked as "non-trusted" this may suggest there is something wrong with CA's certificate. For me it was like that in the old buggy keyChain

@sarslander mhmm, by the way it is a really weired behavior this thing about private keys. I don't know if it might be of help, but I have updated from mojave to catalina when reinstalling the os in my machine. Catalina is the last version, maybe it has updated CAs and stuff in the keyChain.. but it is just my guessing.. I also have installed the current last stable version of Xcode

@Luke-f yes it is frustrating, if I also try to sign it manually with the downloaded distribution profile, xCode says it cannot recognize the certificate inside it even if it can generate the distribution. But when I try lo launch the app manually signed in my iPhone, it crashes at the splash screen

@all yes I had to fully reset the machine and reinstall from scratch macosx catalina os. I have a dedicated machine for doing the building of ios mobile apps so I didn't really care about backups. As for my understading, the old keyChain was pheraps missing some public keys of certain Apple CAs, but the distribution private key was correctly present because it was generated by Xcode. I'm saying this because after the reset, I opened the keyChain and I noticed there were new CA certificates compared to the old bugged keyChain. Moreover, in the bugged keyChain the distribution certificate (with its privateKey inside) was reported as "non-trusted", this might suggest that the OS couldn't decrypt and trust it with any of the CA's public key already present in the keyChain. I don't know if apple has changed some policy and/or updated their CAs because my machine was working well for its purpose and suddenly showed the problem when I generated a new distribution certificate in Xcode. Hope this helps

I fixed it by resetting my macosx machine from scratch. Probably there was something corrupted in the keyChain, or more likely there was something missing in the keyChain and that could not be restored. Resetting from scratch my os fixed it