The ability to create Sections via mobile has been put on hold while we transition section creation on the web from “;” to 'Tab+N". It should be back as soon as we fully implement the new shortcut and complete groundwork to implement Switching from Boards to List View (and vice versa).
You can keep an eye on Create Section feature is missing from our mobile app, I’ll make sure to post an update there as soon as we re-implement section creation via mobile. Sincere apologies for the inconvenience and thank you so much for your patience while we complete this work!