HI - when I'm in the basic interface and set some keyframes for a default cube - I cannot grab the green frame on the timeline and drag it to a new time position (but I see it being done in tutorials). What am I doing wrong? Thank you

If you middle-click on the timeline, a white bracket will appear. You can stretch it by grabbing the tabs on either side to extend the range. Any keyframes inside the range can be middle-dragged if to a new location, preserving the distance between frames. You must initiate the middle-drag anywhere within the tinted range. Alternately, you can middle-click-drag on an individual keyframe to relocate it on the timeline. It briefly turns red while you're dragging.

Also hold down Shift and then use left mouse to make a range selection of keys on the timeline.