Cloud computing (especially AWS). I've spent the last decade working on application software and am increasing worried how many jobs expect cloud experience these days as I have none.
If you are just starting out, i highly recommend Cloud Computing for Science and Engineering by Ian Foster et al. - https://cloud4scieng.org/ It gives a comprehensive overview of the Cloud Architecture, the various pieces involved and how they all work together. The whole "Cloud Technologies" field is quite vast and overwhelming and this book provides a nice comprehensible roadmap.
I've never had a problem with programming languages – I'd pick up a book, do the exercises, play around with the language and get a feel – but I find "cloud" hard to get into for some reason.