I cannot find Valin Hess’ Imperial rank online anywhere. He’s just called an “Imperial officer”, or he’s referred to by his given name.
The Star Wars: Helmet Collection (Part 1) contains a list of Imperial Rank Insignia. It's possible that Hess is a general, but it seems more likely that he was either a Colonel or a Major.
Note that the same rank placard is seen on an Imperial Navy Commodore, but it's passingly unlikely that this would apply.