An eye for design
Obviously, digital marketing is about showing eye candies to your intended reach. You must know basic design elements such as pantones, hex colour codes, typography, among others.
Solid fundamentals in marketing
Digital marketing isn't just creating a catchy poster to share on social media. You also need the fundamentals - AIDA, the BCG matrix, etc. I mean, you can't just launch a boosted tweet without knowing your target's psychographic details, right?
Analytics says it all
And this means you understand the data that's presented to you when you accessed your page insights. How much will you spend this year compared to last year, and what judgment are you making based on whatever data you have? According to your growth rate, what are your next steps? Things like those.