Hi @Petter ,
The error here is due to a data type mismatch: You need to have the 2nd argument and 3rd argument result into the same data type as your target property here CC #.
Since your target property has a Cost Center dimension data type as well you would need to adjust the 2nd argument.
To achieve the logic you are looking to apply the 2nd argument could reference the item directly within the Cost Center dimension directly:
'Cost Center GL'.'CC Concatenated'."99999 Dummy CC"
This would result in the Cost Center dimension data type.
hope this helps,
Issam