It wasn’t clear to me why you couldn’t select, so typically I ask for a scene file. Since that is one of the main parts of the tutorial, I assumed that somewhere else must the problem.
You mentioned the Instant Mesh program, which focused my attention to the problem in a different direction (my fault, but I’m used to interpret anything as the source of a problem, until we find the real source), hence my suggestion to use perhaps a polygon object, to avoid extra work, or an extra source to be the problem. Problem-solving strategy: exclusion.
Typically, you select the object in question. Then the mode, what you like to select, e.g., Edge Mode, is chosen. It should then work with a “cmd/ctrl + a”. [select all]
You will find that command as well as in Main Menu> Edit> Select all. Make certain that the “mouse” is inside the specific window. If the mouse indicator is outside, that area will be considered as target to be selected.
Please let me know if there is any other question.
All the best