Which skill should I learn in digital marketing? What digital marketing skills should I focus on learning?

I would say start with copywriting. It extends to all areas of the digital world. A key element of copywriting is understanding human psychology and perfecting the art of effective communication; understanding copywriting formulas, the power of trigger words, and arranging words in the right order to elicit desired action is an invaluable skill. Mastering copywriting will serve you well in any field of marketing. Even if you choose to specialize in a subfield like analytics, knowledge of consumer psychology will make you a better analytics professional.