You’ll notice that the third item down has PAS at the end of it. This is the name of the Synchronization Connection I setup with a name of "PAS." The way I configured it was to be based off of a BCS connection as seen below.
You can see from the screenshot above that I am connecting to an External Content Type (ETC) called "PAS User." I’m then mapping the ETC to existing profiles via a profile property called "saMAccountNameString." This is a custom profile property I created that has a type of "String" which is mapped to the saMAccountName coming from AD. Not following this approach is a key reason why some people get an error like the following:
Let me take a step back and cover what approach people who get this error are doing. Instead of creating a String based property to match off of they instead try to map to one of the existing fields like "Account Name" or "User Name." The reason neither of these would work is because they are of type "User." This actually makes sense when you look back at the error message.