If your iOS developer and distribution certificates do not appear in My Certificates, then they are not correctly configured for use on your Mac. Please note that "Certificates" is a repository of all certificates your Mac holds, whereas "My Certificates" is the subset of certificates valid for your Mac to actually use - a certificate appearing in "Certificates" only is not enough.
If the certificate is not in My Certificates this is most likely because you do not have the correct key for that certificate also on that Mac. You will need to locate the private key made for that certificate (i.e., from the original Mac which requested the certificate or a backup server). If you no longer have the key it is likely you will need to revoke the certificate and make a new one at developer.apple.com.
As long as they do appear in My Certificates, then they key is there. Now look for any red or blue icons on the certificate's icon. Both of these indicate problems:
- A red X icon certificate will usually tell you quite clearly what the issue is - either expired or revoked - and issues with red certificates are fixed in your control panel at developer.apple.com. You make the new certificate and then get Xcode to refresh your installed certificates in Xcode > Preferences > Accounts.
- A blue + icon certificate means mostly likely that you've set the certificate to always be trusted in its properties. You'd think this would mean the certificate will always be trusted, but it doesn't. Please right-click the certificate and choose Get Info, then set any values back to "Use System Defaults" and okay the changes to fix the issue and remove the +. This is usually accompanied by a NOT_TRUSTED error appearing in your build log.
If there are no icons, then the next step is to key the key attached to the certificate is set to "Always allow access". This is the most common build issue which catches new developers out when developing for iOS.
- Click the expand arrow next to your certificate to expose the key, right-click the key and choose Get Info, then the Access tab and toggle the setting at the top. This is usually accompanied by a USER_INTERACTION_NOT_ALLOWED error appearing in your build log.
Remember that when making changes to your certificates on developer.apple.com or your provisioning profiles on iTunes Connect you will need to have Xcode actually download the changed files. You do this via Xcode > Preferences > Accounts. Your developer details should be in the list of accounts, simply choose to view the info for that account, then click the refresh button on the new window which appears.
If this fails to address the issue for you, we will need to see a full compiler log to show the error you have when trying to build for iOS. Please do not censor any of the username/password details or filenames/folder paths, as this could be vital information relevant to the issue. (Please copy the compiler window log text into a new text document and then attach that document when making a new support ticket.)