If you do the connection the other way around, first add the Redshift, load contents, and then add the Excel, does it work?
I just wonder if you are able to get the Redshift content at all, so making the reverse process may answer that question
I hope this helps
Its possible however in order to perform such join Tableau has to suck in all the underlying data and, depending on the size of your Redshift table(s), it could take some time.
Also, make sure your join condition is valid and does not duplicate rows.