Are you using Windows Internal Database or SQL Server?
So I had this project for a customer lately, and everything was working as inteded until I was expanding the ADFS Farm with a second ADFS Server.
It continuely told me I couldn’t add the Server to the ADFS Farm, because I was using a SQL Database Instance. But I was so sure I haven’t installed any SQL Server or configured the first ADFS Server with any SQL Database Instace.
Even today, I don’t know how this could have happend, but with PowerShell command Get-ADFSproperties we can see wether we use WID or SQL.
SQL Express, which is free and gives more the WID will have this settings: