Just in case you are facing the certificate error while installing Sitecore 9.0 update 2 with SIF 1.2.1 as step CreateSignedCert : NewSignedCertificate, as below:
Install-SitecoreConfiguration : Cannot process argument transformation on parameter 'Signer'. Cannot convert the "System.Object" value of type "System.Object" to type "System.Security.Cryptography.X509Certificates.X509Certificate2".
The issue is coming from having the certificate already in a system, so you might manually delete that those from Microsoft Management Console (mmc.exe from Windows start menu).
In my case, I had several certificates named
DO_NOT_TRUST_SitecoreRootCert installed under
Certificates - Current User - Trusted Root Certification Auhorities- Certificates, deleting them helped progress.
Hope this helps!