Looks like you are using data blending to combine your 2 data sets. When you use blending, any data coming from the secondary data source, in your case [Ventas vs TRPs (YouGov) (Ventas Scorecard)], has to be aggregated and therefore your expression uses a SUM. The problem is CORR can't use an aggregation as one of its parameters, so blending won't work.
Have you tried to combine your 2 data sets using cross database joins instead? if you are able to use that instead of blends, you don't need to use an aggregation like SUM for data on your secondary data set.
These links talk about cross DB join: Integrate your data with cross-database joins in Tableau 10 | Tableau Software , Cross-database Joins
Hope this helps!