The question is not clear if the problem is because the 3rd party mac installer certificate is missing. This is the assumption I am making in my response. I'm running Xcode 10.1 and macOS High Sierra (10.13) on a Mac Mini Server. I'm uploading Unity apps to the Mac App Store. After hours of attempts and no help from Apple Development other than 'read the documentation', I was able to create the two certificates. These are the steps assuming that you have logged into the developer account and have clicked your account link. On the Certificates, Identifiers & Profiles page, click the + to the title Certificates. You should see a page titled Create a New Certificate. Select Mac App Installer, then click the Continue button. You should see a window asking for a Certificate Signing Request file. If you already have one you can click the Choose file link to upload it from your computer. The name of my file is CertificateSigningRequest.certSigningRequest. If you don't have a file, click the Learn More link for instructions on how to create it. Once the file is uploaded, click the Continue button. You should see a page titled Download Your Certificate . 3. Click the Download button. The name of my file is macinstaller.cer. Note where the file is downloaded. 4. Double click the file macinstaller.cer to install it in your Keychain. Log into the Keychain Access to verify that you see and entry named "3rd Party Mac Developer Installer: Yourname (youridnumber)". This process may be different using later versions of Xcode and macOS.