Sitecore9 installation error: Cannot process argument transformation on parameter ‘Signer’. Cannot convert the “System.Object[]” value of type “System.Object[]” to type

Remove existing certificate. If you know the thumbprint use the below powershell command to remove certificate-

Get-ChildItem Cert:\LocalMachine\Root\<<thumbprint>> | Remove-Item

To list the existing certificates use following command-

Set-Location Cert:\LocalMachine\Root
Get-ChildItem | Format-Table Subject, FriendlyName, Thumbprint -AutoSize