The error message is telling you exactly what’s wrong.
- On your remote mac’s dock, open the Launchpad, and open Other
- Open Keychain access
- Select “login” on the left-hand side. You should see your Developer certificate in the list of certificates.
- Drag 'n drop that certificate onto “System”, right below “login”.
- You should be good to go now!
3b) If you don’t see your certificate there, you’ll need to add it by pressing the “+” button on the bottom. Make sure it’s in the System keychain.
EDIT: Woops, you might have tried this.
I recommend exporting directly from your keychain, and moving those files from your mac to the Windows build machine. Make sure they are using the same certificates. Before doing this, clear out any certificates that might be hanging around on your Windows machine. Unreal might be selecting a different one than you’re expecting. I have trouble telling them apart in my Project Settings.