I found a question I'm not able to answer definitively, but I do have advice, and I wanted to leave a comment on the question, but I can't for the life of me find the 'comment' button. Please help!
You need to have 50 reputation before you're allowed to comment on other people's questions. You can always comment on your own questions and answers.
The reputation requirement was introduced years ago to prevent drive-by commenting from people who enter the site from Google and leave irrelevant or unnecessary spam comments on topics. The theory is people who have invested a bit of time on the site helping out are more likely to leave useful comments.
Once you earn 50 reputation, the "add comment" link will be visible at the bottom of a post like so: