Presenting data (use Say it with charts by Gene Zalazny, or something by Stephen Few (Show me the numbers, or Information dashboard design)
Software architecture (including how code talks to databases and APIs; concept of technical debt and how trade-offs are made)
Data networking (OSI model, how it relates to TCP/IP)
Scripting (bash/sed/awk or perl or python)
Coding (in some high level OO or functional language; ideally Python or Java)
Project Management (it’s not your job, but sometimes you have to do it)
Tools PMs should be comfortable using, in order to analyse stuff and/or communicate their ideas:
Something for rough mockups, e.g. Balsamiq
(optional) Something for finer mockups, e.g. Sketch 3
Something for quickly drawing flow charts (PowerPoint or Google Slides will do)
Excel (and basic best practices like highlighting inputs, not mixing inputs and calculations, etc.)
Using Powerpoint or Google Slides) to create slides to support a presentation (not just decks for people to read offline or, worse, a deck that is made for presentation, but is so wordy that people focus on the slides instead of the presenter)
SQL (to help you answer your own questions)
A high-level language (e.g. Python, Ruby, JavaScript, Java)
Google Analytics (or MixPanel or whatever your company uses)