SLA Substring the last 6 characters

Good Day,

In SLA User Defined Formula, I am using an Expression to substring the last 6 Characters in the Source "Supplier Site Name" in Payables SLA.

Expression used: Substring("Supplier Site Name"(AP,S,S),-6,6). This Validates Successfully but currently does not work. This Expression behaves exactly like using the starting Position as 1 and then takes the first 6 characters.

If Anyone can assist me on how to Substring the last 6 characters of the Supplier Site name and advise if this would work weather the Supplier Site name is 10 characters or 20 characters. Substring should be dynamic in always taking the last 6 characters.