I would recommend ditching Bootstrap altogether. It has its uses but often times (especially for beginners) it makes CSS which is already extremely difficult to debug and work with, even more difficult to debug and work with.
One way you can fix it is by adding
justify-content: center; to #author-box.
Flex-box and CSS grid can accomplish a lot of what people use Bootstrap for. If you need a drop in UI and you like Bootstrap, then sure, but if you’re using it for the grid system, it is significantly more complicated, and more brittle than just using flexbox or css grid.